METHOD FOR IDENTIFICATION AND ISOLATION OF HEMATOPOIETIC PROGENITOR CELLS

Abstract: There is provided a method for detecting and/or differentially identifying long-term repopulating hematopoietic progenitor cells. The method comprises detecting expression of tissue transglutaminase, which is uniquely expressed by long-term repopulating cells among blood or marrow cells. Thus, the method can comprise, for example, detecting the expressed protein, detecting an mRNA encoding the protein, or detecting an enzymatic metabolite of the protein. There is also provided a method for isolating long-term repopulating hematopoietic progenitor cells, comprising contacting cells from a sample of blood or marrow with an immobilized probe that selectively binds to tissue transglutaminase, to bind the long-term repopulating cells, and then collecting the bound cells. The selective identification or isolation of such long-term repopulating hematopoietic progenitor cells provides specific advantages, including the potentiation of allograft procedures, marrow reconstitution following myeloablation, and similar serious medical procedures.

Description: METHOD FOR IDENTIFICATION AND ISOLATION OF HEMATOPOIETIC PROGENITOR CELLS BACKGROUND OF THE INVENTION Field of the Invention The invention relates to methods for identifying and isolating hematopoietic cells. More particularly, the invention relates to methods for specifically identifying and isolating early hematopoietic progenitor cells. Description of the Prior Art Each day the bone marrow generates and releases into the circulation several billion fully-differentiated, functional blood cells. Production of these cells derives from a small stock of quiescent progenitor cells (including the most primitive stem cells and other less primitive but still immature progenitors) by a process called hematopoiesis. The basic phenomena of this process of hematopoiesis (alternatively, "hemopoiesis" or "haematopoiesis") are described in the literature. See, e.g., Dexter and Spooncer (1987); Zipori (1992). However, the specific functional and mechanistic aspects underlying the process remain unclear. The most primitive stem cells have two apparently incompatible functions: the maintenance of a life-long capacity to continually maintain the blood cells in the body, and the generation of huge numbers of different types of blood cells on a daily basis. It is estimated that the human hematopoietic system has the capacity to generate >10 13 cells containing all blood lineages (Turhan et al. 1989). The production of such a large number of cells is achieved by extensive proliferation coupled with successive differentiation steps leading to a balanced production of mature cells. During the processes of differentiation, progenitor cells progressively lose their capacity to generate multiple cells lineages and eventually produce cells of one or two cell lineages. Yet, throughout life, some progenitor cells remain that are capable of generating most if not all of the cell types. Soluble regulators and cell-cell interactions mediate differentiation pathways of immature progenitors through a tightly-controlled but inadequately understood process. Several of the body's soluble factors have been isolated and characterized both in culture and in animals (see, e.g., Ogawa (1993) and references therein). Regulators such as the colony stimulating factors (e.g., IL-3, GM-CSF, G-CSF, M-CSF) not only induce proliferation and differentiation of progenitors capable of producing cells of either multiple cell lineages (IL-3 and GM-CSF) or single cell lineages (G-CSF and M-CSF), but also preserve viability of their respective progenitors for short periods. Other regulators such as interleukin-1 (IL-1), the kit ligand (KL), and thrombopoietin (Borge et al. 1996) increase viability of multipotential progenitors in addition to other functions. However, no known cytokines alone or in combination can preserve viability of primitive progenitors in liquid culture without stromal support beyond a few days. Accordingly, functional assays that depend on formation and growth of colonies of cells over days or weeks are performed in the presence of added stromal cells. See, e.g., Hao et al. (1996), etc. Regulation of primitive stem cells appears to differ from that of immature, multilineage progenitors. Hematopoietic "stem" cells, which reside in the bone marrow predominantly in a quiescent state, do not appear to respond immediately to regulators that induce differentiation and proliferation. Maintenance of these cells in the body is mediated via cell-cell interactions and soluble regulators. Maintenance of quiescent progenitor cells in vitro has been achieved by culturing cells on adherent stromal layers with soluble regulators such as IL-3, IL-6, KL and LIF (Young et al. 1996). Recently, the addition of the Flk-2/Flt-3 ligand (FL) to this complex culture has been found to extend maintenance of quiescent progenitor cells from a few weeks to three months (Shah et al. 1996). Various methods are known for use in the effort to differentiate the numerous subpopulations in the hematopoietic systems. Such methods include, for example, expression of cell surface proteins (markers), cell size and density, uptake of fluorescent compounds (e.g., Rhodamine-123), resistance to cytotoxic drugs (e.g., 5-fluorouracil), and adherence to plastic. However, these methods have yet to resolve the true scope of the hematopoietic system, and have not yet proven capable of identifying whether or not a single cell type exists that might be understood to the most primitive hematopoietic progenitor. The most developed system for characterizing the different subtypes of cells in the hematopoietic system is that based on differential expression of cell surface markers. Dozens of these markers are known and cataloged, with various combinations of markers identifying unique subpopulations of cells. In particular, it is generally accepted that the CD34 + cell subset contains virtually all human hematopoietic progenitors that are capable of clonal proliferation or expansion (clonogenic) in vitro (Chen et al. 1997). But it is also generally accepted that the CD34 + population is itself phenotypically and functionally heterogenous. Therefore, it appears that CD34 does not uniquely identify hematopoietic stem cells in the human. To illustrate the complexity of the hematopoietic system, it has been reported, for example, that a population of murine cells exists that expresses the markers Sca-1 and c-kit, that lacks the lineage markers Mac-1, GR-1 B220, IgM, CD3, CD4, CD8, and CD5 (collectively "Lin "), but that could be subdivided on the basis of expression of CD38. Randall et al. (1996). These cells are said to be long-term reconstituting hematopoietic progenitor cells. By the same token, long-term lymphohematopoietic reconstitution in mice is said to have been achieved based on a single CD34 lo/" hematopoietic progenitor cell. Osawa et al. (1996). This cell was also c-kif and Sca-1 + , but Lin " . Another report identifies a lineage of multipotent hematopoietic progenitors in mice based on expression of Thy- 1.1 , Sea- 1 , Lin, c-kit, Mac- 1 , and CD4. Morrison et al. (1997). This report (and others) also notes differences in cell function that appear to correlate with different quantitative levels of expression of some of these markers. Long term reconstituting cells are characterized by Morrison et al. as Mac-1 ~ CD4 " c-fø ' t + . By contrast, Morrison et al. characterize transiently reconstituting cells as Mac-1'°CD4 " or Mac- l'°CD4 10 cells. Other differential methods combine the use of marker characterization along with other differential tests. For example, Spangrude et al. (1995) describe long-term hematopoietic repopulation of mice based on progenitor cells identified as Thy-l.l l0W Lin " Ly-6A/E + and further selected for low retention of the fluorescent compound rhodamine 123 (Rh-123 low ). This reports concludes that two classes of long-term repopulating cells may exist. Zijlmans et al. (1995) also report differentiation of murine long-term vs. short-term repopulating hematopoietic cells on the basis of differential staining with Rh-123. Relatedly, Berardi et al. (1995) demonstrated functional differentiation of cells based on depleting bone marrow of cells responsive to the cytokines, c-kit ligand and IL-3. The remaining population (cells resistant to 5-fluorouracil) was found to be quiescent and was enriched for long-term repopulating cells. Hao et al. (1996) report that there may exist a hierarchy of progenitors — with the more long-term repopulating cells exhibiting a higher degree of quiescence in culture and lower susceptibility to retroviral transduction — that may require study by functional colony forming assay systems. See also Shah et al. (1996), who report that Flt3 ligand induces proliferation and differentiation of highly quiescent progenitor cells. The human STK-1 receptor (or its murine homolog Flt3) may function as a growth factor receptor on these primitive progenitor cells. See Small et al. (1994). Ikuta et al. (1992) disclose that murine repopulating stem cell populations are characterized as Thy-l l0 and Sca-1 + , but Lin " . By contrast, this same paper describes human repopulating stem cell populations as being as CD34 + Lin " . It is clear that numerous cell surface markers have been identified that are informative or identifying for subpopulations of myeloid cells and various other cell types. However, hematopoietic progenitors have not been fully characterized as to the full spectrum of markers expressed on their surfaces, or as to how the families of expressed surface markers shift as differentiation proceeds. Moreover, the functions of many of the identified markers have not been established. In view of the above considerations, it is clear that existing technology is not satisfactorily able to identify and isolate the earliest progenitor cells in the hematopoietic regime. Current protocols are limited in their ability to resolve closely related but functionally distinct cells types. Because of this, the most fundamental and earliest progenitors in the hematopoietic cascade have remained elusive to conventional technologies. As a consequence, numerous potential therapeutic approaches to diseases such as cancer and genetic blood diseases remain poorly explored. Accordingly, it is one of the purposes of this invention to overcome the above limitations in the identification of the earliest hematopoietic progenitor cells, by providing a method which enables the artisan to readily and directly differentiate such cells from later progenitors. The method also permits the artisan to selectively isolate the long-term repopulating progenitor cells with precision and accuracy. As a result, the manipulation of these progenitor cells is rendered more controllable. Therapeutic methods, such as controlled myeloablation procedures with subsequent hematopoietic reconstitution, are rendered more easily accomplished. SUMMARY OF THE INVENTION It has now been discovered that these and other objectives can be achieved by the present invention, which in one embodiment is a method of differentially identifying long-term repopulating hematopoietic progenitor cells, comprising: (a) providing a sample comprising myeloid cells, preferably a sample of bone marrow or blood; (b) measuring tissue transglutaminase expression by cells in the sample; (c) comparing the measured amount of tissue fransglutaminase expression for the sample to a standard value established as a background amount; wherein a measured amount of tissue transglutaminase expression for the sample in excess of an established background amount indicates the presence of long-term repopulating hematopoietic progenitor cells in the sample. The measuring step can comprise measuring an amount of tissue transglutaminase or a unique fragment thereof. For example, the measuring step can comprise measuring in the sample an amount of binding of an antibody having specific binding affinity for tissue transglutaminase. The antibody is selected from the group consisting of polyclonal antibodies, monoclonal antibodies, chimeric antibodies, humanized antibodies, Fab fragments, Fv fragments, and Fab' 2 fragments. It may be preferred that antibody be detectably labeled. Alternatively, the measuring step can comprise measuring an amount of an mRNA encoding a tissue transglutaminase or a unique fragment of the mRNA. The measuring of the amount of the mRNA can comprise hybridizing the mR A with a detectably labeled nucleic acid probe that hybridizes to a nucleic acid encoding tissue transglutaminase under stringent hybridization conditions. Alternatively, the measuring step can comprise measuring an amount of a metabolite of a tissue transglutaminase. For example, the method can be used to measure production of a metabolite that is fluorescent or colored. In another embodiment, the invention is a method of differentially identifying long-term repopulating hematopoietic progenitor cells, comprising measuring tissue transglutaminase expression in a sample comprising myeloid cells, wherein a measured amount of tissue transglutaminase expression for the sample in excess of an established background amount indicates the presence of long-term repopulating hematopoietic progenitor cells in the sample. In another embodiment the invention is a method of selectively isolating long-term repopulating hematopoietic progenitor cells from a blood or marrow sample, comprising: a) providing a probe having specific binding affinity for tissue transglutaminase ; b) contacting cells from the blood or marrow sample with the probe under conditions sufficient to permit binding of the probe to cells expressing tissue transglutaminase; c) separating the cells bound to the probe from the cells not bound to the probe; wherein the separated bound cells are enriched in long-term repopulating hematopoietic progenitor cells. Here, the probe can be linked to a substrate, and in such cases the separating step can comprise removing cells not bound to the probe and retaining cells expressing tissue transglutaminase bound to the substrate. Preferably, the probe is an antibody selected from the group consisting of polyclonal antibodies, monoclonal antibodies, chimeric antibodies, humanized antibodies, Fab fragments, Fv fragments, and Fab' 2 fragments. In a preferred case, the probe can be labeled with a fluorescent moiety, and wherein the separating step comprises selectively collecting the cells bound to the probe by fluorescence activated cell sorting. In still another embodiment, the invention is an assay for detecting long-term repopulating hematopoietic progenitor cells in a sample of cells, comprising: (a) providing a sample comprising blood cells; (b) measuring in the sample an amount of a metabolic product selected from the group consisting of: (i) a tissue transglutaminase or a unique fragment thereof; (ii) an mRNA encoding a tissue transglutaminase or a unique fragment of the mRNA; and (iii) a metabolite of a tissue transglutaminase; and (c) comparing the measured amount of the metabolic product in the sample to an amount established as a background amount; wherein a measured amount of the metabolic product in excess of the established background amount indicates the presence of long-term repopulating hematopoietic progenitor cells in the sample. These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention have been chosen for purposes of illustration and description, but are not intended in any way to restrict the scope of the present invention. The preferred embodiments of certain aspects of the invention are shown in the accompanying drawings, wherein: Figure 1 illustrates, through gene expression fingerprinting, tissue transglutaminase cDNA present in fractions of long-term repopulating hematopoietic progenitor cells from murine bone marrow, and not the fractions of short-term repopulating hematopoietic progenitor cells. Figure 2 illustrates, through semi-Southern blots of two separate control experiments, the distribution of tissue transglutaminase mRNA in fractions of long-term repopulating hematopoietic progenitor cells from murine bone marrow, and not the fractions of short-term progenitor cells. Figure 3 illustrates a semi-Southern blot performed in a third control experiment on fractions separated into long-term repopulating hematopoietic cells, short-term repopulating hematopoietic cells, and an unknown intermediate. Tissue transglutaminase « was only fully expressed in the fractions of long-term repopulating cells. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is directed to a method for isolating or identifying long-term repopulating hematopoietic progenitor cells. The method takes advantage of a newly identified cell marker that characterizes or distinguishes such long-term repopulating progenitor cells from other immature and mature blood cells. Applicants have unexpectedly identified a marker that uniquely identifies long-term repopulating hematopoietic progenitor cells among myeloid cell populations. In particular, it has been found that tissue-type transglutaminase (also known as transglutaminase type II) is expressed in long-term repopulating progenitor cells, but is not measurably expressed in any other tested myeloid cells. In particular, it has been found that the tTGase marker differentially identifies long-term repopulating cells from other closely related progenitors, namely short-term repopulating cells, in assays performed according to the invention. Thus, there has been identified a new and important marker that can be exploited to identify long-term repopulating progenitor cells with precision and accuracy. Transglutaminases (TGase) are a family of enzymes widely distributed in vertebrates. The family of transglutaminases includes a variety of types, for example, secretory, tissue, epidermal, keratinocyte, and hemocyte types, among others. Wilhelm et al. (1996). In particular, tissue-type transglutaminase (tTGase) is a GTP-binding Ca 2+ -dependent enzyme that catalyzes post-translational modification of proteins. Mian et al. (1995). The enzyme is reported to be involved in a number of physiological processes including apoptosis, cell adhesion, metastasis, and extracellular matrix (ECM) assembly. Hettasch et al. (1996). In particular, a number of reports indicate that the tTGase is important in physiological cell death. See, e.g., Piacentini (1995); Cummings (1996). For example, it is reported that tissue-type transglutaminase is involved in the apoptotic cell death associated with embryonic pattern formation. Adel Moallem et al. (1996). The cross-linking function of transglutaminases is thought to be a principal mechanism involved in apoptosis. Fesus et al. (1996). However, none of these reports demonstrates or suggests any role for tTGase in hematopoietic progenitor cells. As used herein, "marrow" refers to the material, including vascular connective tissue, that occupies the central cavities of the long bones in vertebrates, and also the spaces in certain types of cancellated bone. Marrow is sometimes referred to as the "medulla" or the "medulla ossium." The marrow contains "myeloid tissue," which is a hematopoietic tissue responsible for the development of cellular elements of the blood, including leukocytes (white blood cells), erythrocytes (red blood cells), megakaryocytes, platelets, and the like. The term "myeloid tissue" is used herein to refer to any tissue that contains "myeloid cells," i.e., cells that are responsible for or involved in any aspect of hematopoiesis. Myeloid cells include hematopoietic progenitor cells. Accordingly, myeloid tissues include, for example, marrow tissue, blood, spleen tissue, lymph tissue, thymus tissue, and the like. Blood is a commonly sampled myeloid tissue, and can be, for example, peripheral blood, umbilical cord blood, placental blood. As used herein, "hematopoiesis" refers to the process of formation and development of blood cellular elements. It is understood that much of hematopoiesis takes place in the myeloid tissue of the marrow. However, a significant component of hematopoiesis is extramedullary, where blood cells develop and mature outside the marrow, such as in the spleen, liver, and lymph nodes. As used herein, "progenitor cell" refers to any normal myeloid cell that has the capacity to generate fully differentiated, functional progeny by differentiation and proliferation. Progenitor cells include progenitors from any myeloid tissue or organ system, including, but not limited to, marrow, blood, spleen, liver, thymus, tonsil, lymph nodes, and the like. Progenitor cells are distinguished from "differentiated cells," the latter being defined as those cells that may or may not have the capacity to proliferate, i.e., self-replicate, but that are unable to undergo further differentiation to a different cell type under normal physiological conditions. Moreover, progenitor cells are further distinguished from abnormal cells such as cancer cells, especially leukemia cells, which proliferate (self-replicate) but which generally do not further differentiate, despite appearing to be immature or undifferentiated (dedifferentiated). Production of some mature, functional blood cells results from proliferation and differentiation of "unipotential progenitors," i.e., those progenitors that have the capacity to make only one type of blood cell. For red blood cell (erythrocyte) production, a unipotential progenitor called a "CFU-E" (colony forming unit-erythroid) has the capacity to generate two to 32 mature progeny cells. Various other hematopoietic progenitors have been characterized. For example, hematopoietic progenitor cells include those cells that are capable of successive cycles of differentiating and proliferating to yield up to eight different mature hematopoietic cell lineages. At the most primitive or undifferentiated end of the hematopoietic spectrum, hematopoietic progenitor cells include the hematopoietic "stem cells." These rare cells, which represent on the order of from 10 "5 to 10 "4 of the cells in the bone marrow. The most primitive of these cells have the capacity to generate >10 13 mature blood cells of all lineages and are responsible for sustaining blood cell production over the life of an animal. They reside in the marrow primarily in a quiescent state, and may form identical daughter cells through a process called "self-renewal." Accordingly, such uncommitted progenitor cells can be described as being "pluripotent," i.e., both necessary and sufficient for generating all types of mature blood cells. Progenitor cells that retain a capacity to generate all blood cell lineages but that cannot self-renew are termed "multipotent." Cells that can produce two but not all blood lineages and can not self-renew are termed "bipotent." The latter two types of cells may be called "committed" progenitor cells. Human hematopoietic progenitor cells that express the CD34 antigen and/or the Flk-2/Flt-3 receptor are referred to herein as "primitive progenitor cells." Primitive progenitor cells include both pluripotent stem cells and committed progenitors. Therefore, primitive progenitor cells include CD34 + Flk-2/Flt-3 " cells, CD34 " Flk-2/Flt-3 + cells, and CD34 + Flk-2/Flt-3 + cells. By contrast, hematopoietic cells that do not express either the CD34 antigen or the Flk-2/Flt-3 receptor (i.e., CD34 " Flk-2/Flt-3 " cells) are not typically referred to as "progenitor cells," but this group comprises all mature cells and cells at intermediate stages of lineage differentiation. By "long-term repopulating cells" (also abbreviated "LTR-HPC") is meant those cells that enable long-term hematopoietic reconstitution or cells that enable late engraftment. Such cells are a subset of the hematopoietic progenitor cells as that term is conventionally understood. This subset consists primarily of primitive cells that are relatively quiescent and relatively unresponsive to cytokines. These cells are also relatively resistant to efficient retroviral transduction. Hao et al. (1996). By "short-term repopulating cells" (also abbreviated "STR-HPC") is meant those cells that enable short-term hematopoietic reconstitution or that enable early engraftment. Such cells are a complementary subset of the hematopoietic progenitor cells. This subset consists primarily of somewhat less primitive cells that are relatively less quiescent and relatively more responsive to cytokines. These cells are also relatively more susceptible to efficient retroviral transduction. As a result of the present invention, numerous utilities become technically feasible. The method of the invention can be performed in vitro, ex vivo, or in vivo. "In vitro " methods include methods such as laboratory experimental methods which are performed wholly outside a living body. While cells can be acquired from a living organism for use in vitro, it is understood that the cells will not be returned to the body. In vitro methods are commonly employed in experimental settings to advance understanding of particular systems. "Ex vivo" methods include clinical methods in which cells are manipulated outside the body of an organism, e.g., a patient, with the specific purpose of reimplanting some cells back into the organism to achieve a desired therapeutic purpose. "In vivo" methods are performed within the body of the organism, without requiring explantation or tissue sampling and manipulation. In one embodiment, the invention is a method of differentially identifying long-term repopulating hematopoietic progenitor cells. For example, the method can comprise providing a sample comprising myeloid cells, measuring a parameter characteristically associated with tissue transglutaminase expression by cells in the sample, and comparing the measured value of the parameter for the sample to a standard value established as a background amount. In this embodiment, a measured value of the parameter for the sample in excess of an established background value indicates the presence of long-term repopulating hematopoietic progenitor cells in the sample. A measurable parameter associated with or characteristic of tTGase expression or activity in hematopoietic progenitor cells can be employed according to the method of the invention. Any measurable parameter can be used, including biological, chemical, and physical parameters. Such parameters include, for example, tTGase mRNA expression, intracellular tTGase expression, cell surface tTGase expression, intracellular tTGase activity, extracellular tTGase activity, and the like. For example, the measuring step can comprise measuring an amount of tissue transglutaminase or a unique fragment thereof on surfaces of myeloid cells, or in a cell-free system (e.g., lysate, homogenate, etc.). Alternatively, the measuring step can comprise measuring an amount of an mRNA encoding a tissue transglutaminase, or a unique fragment of the mRNA. In another alternative approach, the measuring step can comprise measuring an amount of a tTGase substrate or a metabolite. Methods are known for assaying various parameters of tTGase activity or expression in other systems, and these methods can be readily adapted by the artisan for use according to the invention. Thus, in a preferred embodiment, the invention is a method of differentially identifying long-term repopulating hematopoietic progenitor cells, comprising measuring tissue transglutaminase expression in a sample comprising myeloid cells, wherein a measured amount of tissue transglutaminase expression for the sample in excess of an established background amount indicates the presence of long-term repopulating hematopoietic progenitor cells in the sample. As noted herein, Applicants have identified an mRNA that encodes tTGase, which is produced by LTR-HPC but which is not produced, or is produced in insignificant quantities, by other myeloid cells. Accordingly, the invention includes a method of selectively identifying LTR-HPC on the basis of production of this mRNA. This can be accomplished by conventional methods such as the reverse transcription-polymerase chain reaction method (RT-PCR). RT-PCR is a two step technique that comprises reverse transcribing a mRNA into complementary DNA (cDNA), and then amplifying the cDNA using the standard polymerase chain reaction. The amplified product can then be resolved by conventional electrophoretic techniques. This technique is qualitative as described, but can be rendered quantitative by incorporation of a competing template as an internal control. This latter technique is termed "competitive RT-PCR." Alternatively, in situ hybridization techniques can be used to define cell populations expressing tTGase. See, e.g., Gentile et al. (1994). Northern blot analysis can be performed using oligonucleotides that hybridize specifically to nucleic acids encoding tTGase. See, e.g., van Groningen et al. (1996). In this type of assay, total RNA is isolated from a cell sample, and then contacted with a specific probe of interest under hybridization conditions. A measurable amount of hybridization of the probe to the target sequence is an index of target sequence expression in the cell sample. Thus, in the present case, this technique can be adapted to measure hybridization of a tTGase-specific probe to total RNA isolated from a myeloid cell-containing sample. Measurable hybridization will indicate the presence of LTR-HPC in the sample. The presence of LTR-HPC in a sample comprising myeloid cells can be determined using conventional antibody-based detection techniques, such as enzyme-linked immunosorbence assays (ELISA), radioimmunoassay (RIA), immunoblotting, immunoprecipitations, and the like. Immunoblotting can be performed generally in accordance with the method described by Takaku et al. (1995). Briefly, a cell sample is homogenized, and cellular proteins are isolated from a centrifugal supernatant. The proteins are then separated by polyacrylamide gel electrophoresis (SDS-PAGE) and then transferred to a membrane (typically nitrocellulose). The membrane is then incubated with an anti-tTGase antibody, washed and then incubated with a secondary labeled antibody. If the label is horseradish peroxidase (HRP), for example, detection of binding to the immobilized protein is based on measurable production of a chemiluminescent metabolite of HRP. Kits for such chemiluminescence-based assays are commercially available, e.g., the ECL kit from Amersham. Alternatively, the presence of LTR-HPC in a tissue or cell sample can be determined with great sensitivity by means of immuno-PCR techniques. See Sano et al. (1992). In this type of technique, an antibody that specifically binds to tTGase is detectably labeled with a marker nucleic acid, and then contacted with a tissue or cell sample under conditions suitable for binding of the antibody to any tTGase expressed by the cells. (Alternatively, an labeled antibody specific for a metabolite of tTGase can employed.) Then amplification of the marker nucleic acid is effected under conditions suitable for PCR amplification. The resulting PCR products can be separated by electrophoresis and then quantitated by ethidium bromide staining. The presence of PCR products indicates the presence of LTR-HPC in the sample. Immunoaffmity procedures can be used to select and isolate the LTR-HPC from myeloid tissue samples based on differential binding of tTGase expressed by the LTR-HPC. Such techniques involve the binding of an anti-tTGase antibody to a solid or gel support matrix, and then contacting cells with the immobilized antibody to permit selective binding of cells expressing tTGase. Other cells in the sample are removed, leaving immobilized LTR-HPC, which are in turn eluted under conditions sufficient to overcome the attractive forces between the tTGase and the antibody. Cell sorting techniques can be employed to selectively isolate the LTR-HPC population from myeloid cell samples. For example, by selectively labeling the LTR-HPC cells with an anti-tTGase antibody labeled with a fluorescent marker such as fluorescein, fluorescence activated cell sorting techniques can be used to isolate cells tagged with the antibody. These types of techniques are well understood to enable screening and selection of cells based on two or more markers, such that sorting on the basis of tTGase expression can be quantitated relative to expression of one or more other markers. In another embodiment, the invention is a method for determining the presence of LTR-HPC in a sample by detecting activity of expressed tTGase in the sample. The enzymatic function of tTGase is well-defined, involving cross-linking of a primary amine and a peptide-bound glutamine. A number of assays of TGase activity are established in the art. See, e.g., Wilhelm et al. (1996). For example, it is known that tTGase activity can be measured by incorporation of [ 14 C]putrescine into N,N-dimethylated casein by a cell lysate (centrifugal supernatant derived from a cell homogenate). Takaku et al. (1995). In this embodiment, a sample comprising myeloid cells is assayed for tTGase activity, with measurable tTGase activity being correlated with the presence of tTGase-expressing LTR-HPC in the sample. Alternatively, in this embodiment, the invention is a method for determining the presence of LTR-HPC in a cellular sample. For example, a fluorescent or colored metabolite of tTGase can be detected microscopically following incubation of cells with an appropriate substrate of tTGase. See, for example, Verderio et al. (1998), and Lajemi et al. (1997), which describe the use of a fluorescent substrate probe for assessment of transglutaminase activity. The selective staining of the LTR-HPC population enables the artisan to differentiate and if desired quantitate the population in myeloid cell samples. For example, the number of LTR-HPCs in a sample of blood cells or marrow can be determined by marking the cells with a tTGase-specific probe labeled with a fluorescent or colored compound (dye or stain) and then counted using a hemacytometer. However, because the LTR-HPC population is so small (typically about 0.005% of all bone marrow cells), the counting is preferably accomplished using automated techniques, such as Coulter counters, or flow cytometry. As noted, the invention also includes methods for detecting nucleic acids encoding a tissue transglutaminase. Such methods rely on the use of highly specific fragments of nucleic acid as primers and probes in molecular techniques. Nucleic acid and peptide sequences for several tTGases are known in the scientific literature, having been described, for example, by Gentile et al. (1991), Fraij et al. (1992), and Fraij et al. (1996). These publications describe homologs of human tTGase, which are encoded by nucleic acids having the sequences given, respectively, as SEQ ID NO:l, SEQ ID NO:3, and SEQ ID NO:5. The proteins encoded by these nucleic acids have the amino acid sequences given, respectively, as SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6. Homologs in other species are known, such as a murine tTGase encoded by a nucleic acid having the sequence given as SEQ ID NO:7, and the amino acid sequence for this murine tTGase protein given as SEQ ID NO:8 (Gentile et al. 1991). Other tTGase homologs in other species have also been described and published. Given such sequence information, the artisan is equipped with information necessary for manufacture of selected fragments of the sequences for use as specific primers and probes as desired. Techniques for manufacturing and using such nucleic primers and probes are well documented in the art. See, for example, Sambrook et al. (1989), Keller et al. (1993), and Hames et al. (1995). The nucleic acid fragment can be used as a primer ("amplimer") to selectively amplify nucleic acid, such as genomic DNA, total RNA, etc. The fragment can also be an oligonucleotide complementary to a target nucleic acid molecule, i.e., the fragment can be a probe. In either case, the oligonucleotide can be RNA or DNA. The length of the oligonucleotide is not critical, as long as it is capable of hybridizing to the target molecule. The oligonucleotide should contain at least 6 nucleotides, preferably at least 10 nucleotides, and, more preferably, at least 15 nucleotides. There is no upper limit to the length of the oligonucleotide. Longer fragments are more difficult to prepare and require longer hybridization times. Therefore, the oligonucleotide should not be longer than necessary. Normally, the oligonucleotide will not contain more than 50 nucleotides, preferably not more than 40 nucleotides, and, more preferably, not more than 30 nucleotides. Methods for determining whether a probe nucleic acid molecule recognizes a specific target nucleic acid molecule in a sample are known in the art. Generally, a labeled probe that is complementary to a nucleic acid sequence suspected of being in a sample is prepared. Preferably, the target nucleic acid molecule is immobilized. The presence of probe hybridized to the target nucleic acid molecule indicates the presence of the nucleic acid molecule in the sample. Examples of suitable assay methods are described in Dallas et al. (1975); Grunstein et al. (1975); U.S. Patent No. 4,731,325, U.S. Patent No. 4,683,195, U.S. Patent No. 4,882,269, and PCT publication WO 90/01069, all of which are incorporated herein by reference. A measure of homology between two nucleic acids is whether the nucleic acids hybridize with one another. A higher degree of homology is shown if the hybridization occurs under more stringent hybridization conditions. Control and selection of hybridization conditions, and the relationships between hybridization conditions and degree of homology are understood by those skilled in the art. See, e.g., Sambrook et al. (1989). A nucleic acid probe useful according to the invention is any nucleic acid that hybridizes under stringent hybridization conditions with a nucleic acid encoding a tissue transglutaminase. The probes described above can be detectably labeled in accordance with methods known in the art. In general, the probe can be modified by attachment of a detectable label moiety to the probe, or a detectable probe can be manufactured with a detectable label moiety incorporated therein. The detectable label moiety can be any detectable moiety, many of which are known in the art, including radioactive atoms, electron dense atoms, enzymes, chromogens and colored compounds, fluorogens and fluorescent compounds, members of specific binding pairs, and the like. Methods for labeling oligonucleotide probes have been described, for example, by Leary et al. (1983); Renz and Kurz (1984); Richardson and Gumport (1983); Smith et al. (1985); Meinkoth and Wahl (1984). Other methods for labeling nucleic acids are described, for example, in U.S. Patent Nos. 4,711,955, 4,687,732, 5,241,060, 5,244,787, 5,328,824, 5,580,990, and 5,714,327, each of which is incorporated herein by reference. Immunologic techniques such as those described above require the use of antibodies having specific binding affinity (specificity) for the desired target antigen. Typically these antibodies are functionalized to accomplish particular purposes, such as to demonstrate the presence of binding to antigen in a sample. Methods for labeling antibodies have been described, for example, by Hunter et al. (1962) and by David et al. (1974). Additional methods for labeling antibodies have been described in U.S. Patent Nos. 3,940,475 and 3,645,090. Regardless of whether the probe is a nucleic acid or an antibody, the probe may be labeled according to any technique known in the art. For example, the label moiety may be radioactive. Some examples of useful radioactive labels include 32 P, 125 1, 131 I, and 3 H. Use of radioactive labels have been described in U.K. patent document No. 2,034,323, U.S. Patent Nos. 4,358,535, and 4,302,204, each incorporated herein by reference. Some examples of non-radioactive labels include enzymes, chromogens, atoms and molecules detectable by electron microscopy, and metal ions detectable by their magnetic properties. Some useful enzymatic labels include enzymes that cause a detectable change in a substrate. Some useful enzymes (and their substrates) include, for example, horseradish peroxidase (pyrogallol and o-phenylenediamine), beta-galactosidase (fluorescein beta-D-galactopyranoside), and alkaline phosphatase (5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium). The use of enzymatic labels has been described, for example, in U.K. 2,019,404, EP 63,879, and by Rotman (1961). Useful detectable label moieties include, for example, fluorescent, phosphorescent, chemiluminescent, and bioluminescent molecules, as well as dyes. Some specific colored or fluorescent compounds useful in the present invention include, for example, fluoresceins, coumarins, rhodamines, Texas red, phycoerythrins, umbelhferones, LUMINOL®, and the like. Chromogens or fluorogens, i.e., molecules that can be modified (e.g., oxidized) to become colored or fluorescent or to change their color or emission spectra, are also capable of being incorporated into probes to act as reporter moieties under particular conditions. The label moieties may be conjugated to the probe by methods that are well known in the art. The label moieties may be directly attached through a functional group on the probe. The probe either contains or can be caused to contain such a functional group. Some examples of suitable functional groups include, for example, amino, carboxyl, sulfhydryl, maleimide. isocyanate, isothiocyanate. Alternatively, label moieties such as enzymes and chromogens may be conjugated to antibodies or nucleotides by means of coupling agents, such as dialdehydes, carbodiimides, dimaleimides, and the like. The label moiety may also be conjugated to the probe by means of a ligand attached to the probe by a method described above and a receptor for that ligand attached to the label moiety. Any of the known ligand-receptor binding pair combinations is suitable. Some suitable ligand-receptor pairs include, for example, biotin-avidin or -streptavidin, and antibody-antigen. The biotin-avidin combination may be preferred. The probe may be an antibody, such as a polyclonal antibody, but is preferably a monoclonal antibody. The antibodies may be prepared according to methods known in the art. For example monoclonal antibodies may be prepared in general accordance with the method of Kohler and Milstein (1975) and by Campbell (1985); as well as the recombinant DNA method described by Huse et al. (1989). Chimeric and humanized antibodies are also described in the art. Accordingly, the antibody may be selected from among polyclonal antibodies, monoclonal antibodies, chimeric antibodies, humanized antibodies, Fab fragments, Fv fragments, Fab' 2 fragments, and the like. Assays for detecting the presence of proteins with antibodies have been previously described, and follow known formats, such as standard blot and ELISA formats. These formats are normally based on incubating an antibody with a sample suspected of containing the protein and detecting the presence of a complex between the antibody and the protein. The antibody is labeled either before, during, or after the incubation step. The protein, or the cell carrying the protein, is preferably immobilized prior to detection. Immobilization may be accomplished by directly binding the protein to a solid surface, such as a microtiter well, or by binding the protein to immobilized antibodies. In each of the above assays, the sample containing antigen, solid phase immunoabsorbent with immobilized antibody and labeled soluble antibody are incubated under conditions and for a period of time sufficient to allow antigen to bind to the immobilized antibodies and to the soluble antibodies. In general, it is desirable to provide incubation conditions sufficient to bind as much antigen as possible, since this maximizes the binding of labeled antibody to the solid phase, thereby increasing the signal. The specific concentrations of labeled and immobilized antibodies,, the temperature and time of incubation, as well as other such assay conditions, can be varied, depending upon various factors including the concentration of antigen in the sample, the nature of the sample and the like. Those skilled in the art will be able to determine operative and optimal assay conditions for each determination by employing routine experimentation. There are many solid phase immunoabsorbents which have been employed and which can be used in the present invention. Well known immunoabsorbents include beads formed from glass, polystyrene, polypropylene, dextran, nylon, and other material; and tubes formed from or coated with such materials, and the like. The immobilized antibodies may be covalently or physically bound to the solid phase immunoabsorbent, by techniques such as covalent bonding via an amide or ester linkage or by absorption. The invention further enables the artisan to control and manipulate the quiescence and proliferation of hematopoietic progenitor cells by controlling the expression or activity of tTGase by means of tTGase-specific drugs. For example, competitive and non-competitive inhibitors and antagonists of tTGase can be employed to inhibit tTGase activity in these cells. Alternatively, neutralizing techniques can be employed such as using a tTGase-specific antibody (polyclonal or monoclonal) to bind and neutralize expressed tTGase on the surfaces of long-term repopulating progenitors. In another approach, antisense technology can be employed to selectively hybridize to genomic DNA or mRNA to inhibit expression of tTGase in these cells. The artisan will recognize that other related technologies can also be employed to accomplish the inhibition of tTGase expression or activity in hematopoietic progenitor cells. » In another embodiment, there is provided a method of screening putative pharmaceutical agents for their ability to affect long-term repopulating hematopoietic cells. Thus, a parameter of tTGase expression or activity may be assayed as a measure of cellular status, with up-regulation or down-regulation of the parameter of indicating efficacy of the agent being tested. To illustrate, the level of expression of mRNA encoding a tTGase in cells in in vitro culture may be employed as a measure of pharmaceutical activity of an agent. For example, a low level of tTGase expression may indicate either specific or non-specific toxicity of an agent. As a result, the practitioner is provided information pertinent to determining the safety or efficacy of a proposed pharmaceutical agent. Alternatively, there is provided a method for comparative assessment of hematopoietic competence in a subject, wherein an amount of tTGase expression or activity in a hematopoietic progenitor cells from a subject is measured and compared to an amount established as normal for the population. For example, an abnormally low value of measured tTGase activity or expression may indicate a deficiency in the capacity of the subject to repopulate bone marrow. Such information would be valuable to the medical practitioner in aiding understanding and managing disease states and in providing guidance in prescribing therapeutic interventions. Myeloid cells can be cultured in vitro, in conjunction with growth factors such as colony stimulating factors (CSFs) (e.g., IL-3, GM-CSF, G-CSF, M-CSF), interieukins (e.g., IL-1 through IL-18) and KL, to selectively induce proliferation and terminal differentiation of mature progenitors while preserving a significantly enriched population of primitive progenitors. U.S. Patent Nos. 5,472,867 and 5,186,931 describe representative methods of using CSFs and interieukins (ILs) to expand progenitor cell populations in the contexts of, respectively, cancer chemotherapy and bone marrow transplants. The method of the invention can be used to improve functional selection of progenitor cells. Functional selection methods, include the method reported by Berardi et al. (1995) where selection is made using a combination of KL and IL-3 with 5-FU. For example, a process of in vitro selection can be used that involves isolating long-term repopulating progenitor cells, culturing (optionally along with stromal cells) and maintaining the cells in a substantially quiescent state, while using a cytotoxic agent that exhibits selective toxicity for proliferating cells, e.g., to kill cells passing through cell cycle ("cycling cells"). Suitable cytotoxic agents include, for example, compounds such as adriamycin, cyclophosphamide, taxol or other taxane, cisplatin, 5-fluorouracil, and the like. Ex vivo purging protocols can be used to selectively eliminate neoplastic cells by targeting the elimination of actively cycling cells. Once the progenitors cells have been purged of malignant cycling cells, they can be returned to the patient, and permitted to resume normal proliferation and differentiation. In one especially useful scenario, the tTGase marker enables functional selection of normal LTR-HPC from a leukemic bone marrow. Such functional selection and purification of primitive progenitor cells can also be used to enable allogeneic transplant procedures. In situations where autologous tissue is not available for transplant, the method of the invention permits selecting allogeneic long-term repopulating progenitors according to the invention, resulting in the selective depletion of T lymphocytes and other effector cells. This enables transplant of progenitors while inhibiting a graft versus host (GvH) reaction. In this situation, the progeny of the LTR-HPC acquire within the recipient immunological tolerance of the recipient's histocompatibility antigens. The method of the invention further facilitates ex vivo manipulation of hematopoietic progenitor cells for use in gene therapy by enabling selective isolation and identification of long-term repopulating progenitors in liquid culture. By selecting the desired subpopulation, the method enables increased targeting efficiency by viral vectors that enter non-replicating cells (e.g., vectors such as adeno-associated viruses). Thus, in another embodiment, the method can be used in conjunction with methods of transfecting an exogenous DNA sequence into somatic cells. The method can then include transfecting progenitor cells selectively isolated according to the invention. Thus, the method can involve treating a mammalian subject in need of hematopoietic therapy. In particular, the tTGase marker can be used to more efficiently quantify a mammal's hematopoietic competence, i.e., the mammal's ability to generate functional mature blood elements. For example, a tissue sample including hematopoietic progenitor cells can be obtained from the subject. Then the tissue sample can be cultured ex vivo in a growth medium and necessary growth factors and/or stromal cells, while allowing cycling cells to proliferate, differentiate and die. The cultured cells can then be differentially isolated to provide a population significantly enriched in the LTR-HPC progenitor cells. Meanwhile, the mammal can be subjected to conditions sufficient to effect myeloablation, e.g., bone marrow irradiation, whole body irradiation, or chemically-induced myeloablation. Finally, the LTR-HPC-enriched cultured cells can be administered or transfused to the mammal following the myeloablation to generate blood cellular components in the mammal, thereby reconstituting the hematopoietic system of the mammal. The method can use any myeloid tissue sample such as, for example, peripheral blood, umbilical cord blood, placental blood, or bone marrow. Preferably, the tissue sample is autologous to the mammal. While described here as an autologous procedure, the skilled practitioner will recognize that the methods can be readily adapted to transplant of LTR-HPC-enriched myeloid cells from one individual to another. Again, when autologous tissue is not available for transplant, allogeneic cells can be cultured, followed by selective isolation of progenitor cells with concomitant depletion of T lymphocytes and other effector cells. The transplanted LTR-HPC acquire within the recipient immunological tolerance of the recipient's histocompatibility antigens, thereby mitigating graft versus host reactions. Other therapeutic utilities also present themselves to the skilled practitioner as being enabled by the invention. Such other utilities include, for example, expanding LTR-HPC populations ex vivo to increase chances of engraftment, improving conditions for transporting and storing progenitor cells, and removing a fundamental barrier to enable gene therapy to treat and cure a broad range of life-limiting hematologic diseases such as sickle cell anemia and thalassemia. The following examples are provided to assist in a further understanding of the invention. The particular materials and conditions employed are intended to be further illustrative of the invention and are not limiting upon the reasonable scope thereof. EXAMPLE 1 CD34 + and CD34 " mouse bone marrow cells were isolated by cell sorting. The CD34 + and CD34 " fractions were further subdivided to isolate the cells that were also Lin " , Sca-1 + , and c-kif. Sorting was performed in general accordance with the method of Osawa et al. (1996), modified to employ a single laser/dual sort setup. First, bone marrow cells were collected, and the low density fraction was separated by equilibrium density centrifugation on FICOLL®. The cells were incubated with a cocktail of antibodies to mature antigens (Ly-6C, Gr-1, TER119, B220, CD3, and CD8), and separated by immunoaffinity-based magnetic bead separation. The collected cells, i.e., the cells that did not bind to the beads, were incubated with streptavidin/FITC, and further purified by flow sorting. Lineage-marker/FITC -negative cells were then incubated with anti-CD34/FITC, anti-c-fot/Cy-Chrome®, and anti-Ly-6A/E(Sca-l)/PE. CD34 + and CD34 " cells were separated from c-kif, Ly6A + cells in the "lymphocyte/blast" light scatter window. This method yielded numbers of CD34 + and CD34 " cells similar to the numbers described by Osawa et al. (1996). Osawa et al. showed that the CD34 " cells provide long-term in vivo repopulation. We confirmed this with our sorted cells. We also applied the Cobblestone Area Forming Cell assay (CAFC, Breems et al. 1994) to the sorted cells. The in vitro characteristics of the CD34 " and CD34 + cells on stromal cultures support the conclusion that the murine stem cells are in fact CD34 " . These isolated CD34 " and CD34 + cells were used for gene expression fingerprint analysis following the general procedure of Ivanova and Belyavsky (1997). The gene expression fingerprinting method (GEF) enables identification of genes that are differentially expressed between two or more cell types or tissue types. This method is based on consecutive restriction endonuclease digestion of 3 '-terminal cDNA fragments followed by resolution of the fragments on denaturing polyacrylamide gels. RNA from 20,000 CD34 " cells and 20,000 CD34 + cells was isolated using TRIZOL reagent (GIBCO-BRL). cDNA synthesis, amplification by PCR, and restriction digestion were carried out as described in Ivanova and Belyavsky (1997). Nlalll was used for restriction digestion and four different primers (G,A,T,C,) were used to split total cDNA into less complex subsets. One of 30 differentially expressed » fragments was identified as 100 bp 3 '-terminal portion of murine tissue transglutaminase mRNA through determining the degree of homology of the sequence with known tTGase sequence. Figure 1 depicts the band, indicated by an arrow, which corresponds to the 3'- fragment of tissue transglutaminase present in the lanes containing the CD34 " (LTR- HPC) fraction and not the CD34 + (STR-HPC) fraction. EXAMPLE 2 This finding was confirmed by hybridizing semi-Southern blots obtained from cDNA from cells sorted into Lin " , Sca-1 + , c-kit, CD34 " (LTR-HPC) and Lin", Sca-1 + , c-kit, CD34 + (STR-HPC) fractions as described above. The protocol for cDNA synthesis followed the first steps of general GEF protocol described above up to restriction digestion. See, Ivanova and Belyavsky (1997). PCR amplification material was ethanol-precipitated, dissolved in water, loaded onto 2.5% alkali agarose gel and transferred onto Hybond-N+ nylon membrane. Cloned 100 bp fragment of transglutaminase gene, obtained in GEF analysis was used as a hybridization probe. Figure 2 demonstrates the distribution of tissue transglutaminase mRNA in stem cell-enriched sorted fractions of murine bone marrow on the semi-Southern blots described above. Two separate control experiments were conducted utilizing the same protocol, identified as "1 " and "2" in Figure 2. In each experiment, the two lanes containing the CD34 " fraction (LTR-HPC) were designated "-" and the two lanes containing the CD34 + fraction (LTR-HPC) was designated "+". The tissue transglutaminase mRNA was only observed in the LTR-HPC. EXAMPLE 3 A third control experiment was conducted utilizing the same protocol as in Example 2. Cells were sorted into Lin " , Sca-1 + , c-kit, CD34 " (LTR-HPC); Lhr, Sca- 1 + , c-kit, CD34 + (STR-HPC) and Lin " , Sca-1 + , c-kit, CD34" /+ (unknown intermediate) fractions as described above. The functional properties of CD34 " + are unknown and not of particular importance to the experiment. The results of the third control experiment, shown in Figure 3, again demonstrate the tissue transglutaminase fully observed only in the LTR-HPC fraction and not demonstrated in the STR-HPC fraction. Thus, while there have been described what are presently believed to be the preferred embodiments of the present invention, those skilled in the art will realize that other and further embodiments can be made without departing from the spirit of the invention, and it is intended to include all such further modifications and changes as come within the true scope of the claims set forth herein. 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Claims: CLAIMS: 1. A method of differentially identifying long-term repopulating hematopoietic progenitor cells, comprising: (a) providing a sample comprising myeloid cells; (b) measuring tissue transglutaminase expression by cells in the sample; (c) comparing the measured amount of tissue transglutaminase expression for the sample to a standard value established as a background amount; wherein a measured amount of tissue transglutaminase expression for the sample in excess of an established background amount indicates the presence of long-term repopulating hematopoietic progenitor cells in the sample. 2. A method according to Claim 1, wherein the measuring step comprises measuring an amount of tissue transglutaminase or a unique fragment thereof. 3. A method according to Claim 2, wherein said measuring step comprises measuring in the sample an amount of binding of an antibody having specific binding affinity for tissue transglutaminase. 4. A method according to Claim 3, wherein the antibody is selected from the group consisting of polyclonal antibodies, monoclonal antibodies, chimeric antibodies, humanized antibodies, Fab fragments, Fv fragments, and Fab' 2 fragments. 5. A method according to Claim 3, wherein the antibody is detectably labeled. 6. A method according to Claim 1 , wherein the measuring step comprises measuring an amount of an mRNA encoding a tissue transglutaminase or a unique fragment of the mRNA. 7. A method according to Claim 6, wherein the measuring of the amount of the mRNA comprises hybridizing the mRNA with a detectably labeled nucleic acid probe that hybridizes to a nucleic acid encoding tissue transglutaminase under stringent hybridization conditions. 8. A method according to Claim 1, wherein the measuring step comprises measuring an amount of a metabolite of a tissue transglutaminase. 9. A method according to Claim 4, wherein the metabolite is fluorescent or colored. 10. A method according to Claim 1, wherein the sample is a sample of marrow or blood. 11. A method of differentially identifying long-term repopulating hematopoietic progenitor cells, comprising measuring tissue transglutaminase expression in a sample comprising myeloid cells, wherein a measured amount of tissue transglutaminase expression for the sample in excess of an established background amount indicates the presence of long-term repopulating hematopoietic progenitor cells in the sample. 12. A method of selectively isolating long-term repopulating hematopoietic progenitor cells from a blood or marrow sample, comprising: a) providing a probe having specific binding affinity for tissue transglutaminase ; b) contacting cells from the blood or marrow sample with the probe under conditions sufficient to permit binding of the probe to cells expressing tissue transglutaminase; c) separating the cells bound to the probe from the cells not bound to the probe; wherein the separated bound cells are enriched in long-term repopulating hematopoietic progenitor cells. 13 A method according to Claim 12, wherein the probe is linked to a substrate, and wherein the separating step comprises removing cells not bound to the probe and retaining cells expressing tissue transglutaminase bound to the substrate. 14. A method according to Claim 12, wherein the probe is an antibody selected from the group consisting of polyclonal antibodies, monoclonal antibodies, chimeric antibodies, humanized antibodies, Fab fragments, Fv fragments, and Fab' 2 fragments. 15. A method according to Claim 12, wherein the probe is labeled with a fluorescent moiety, and wherein the separating step comprises selectively collecting the cells bound to the probe by fluorescence activated cell sorting. 16. An assay for detecting long-term repopulating hematopoietic progenitor cells in a sample of cells, comprising: (a) providing a sample comprising blood cells; (b) measuring in the sample an amount of a metabolic product selected from the group consisting of: (i) a tissue transglutaminase or a unique fragment thereof; (ii) an mRNA encoding a tissue transglutaminase or a unique fragment of the mRNA; and (iii) a metabolite of a tissue transglutaminase; and (c) comparing the measured amount of the metabolic product in the sample to an amount established as a background amount; wherein a measured amount of the metabolic product in excess of the established background amount indicates the presence of long-term repopulating hematopoietic progenitor cells in the sample.

Inventors: IVANOVA NATALIA B, VISSER JAN W M, IVANOVA, NATALIA, B., VISSER, JAN, W., M.
Applicants: NEW YORK BLOOD CENTER INC, THE NEW YORK BLOOD CENTER, INC.
Application year: 1999
Main classification: C12Q1
IPC codes: C12Q1, G01N33