HUMAN MEG-CSF PROTEIN AND METHODS

Abstract: An isolated human megakaryocyte-colony stimulating factor (hMeg-CSF protein) preferably purified to homogeneity from urine of aplastic anemia patients is disclosed. The novel hMeg-CSF protein is a basic protein having a pI equal to about 7.2-7.4 as determined by isoelectric focusing and a molecular weight of about 29.000-34.000 daltons as determined by SDS-PAGE when in a glycosylated and sialyated form. The novel hMeg-CSF protein has the ability to induce the formation megakaryocyte-colony forming units in a murine fibrin clot assay in vitro, and the further ability to regulate megakaryocytopoiesis and platelet production in vivo. The hMeg-CSF protein can be further characterized as a glycoprotein having biantennary carbohydrate structures, beta-galactose residues as terminal or penultimate sugars and sialic acid moieties. The hMeg-CSF protein is believed to have a specific activity of at least about 4.000 CFU-Meg colonies/mg protein in murine fibrin clot assay. Unique pharmaceuticals containing the hMeg-CSF protein for treating animals including humans and methods for isolating the novel hMeg-CSF protein at various levels of purity are also disclosed.

Description: HUMAN EG-CSF PROTEIN AND METHODS FIELD OF THE INVENTION This application is a continuation-in-part of U.S. Patent Application Serial No. 547,573 filed July 2, 1990. The entire disclosure of that application is incorporated by reference ' in its entirety. This invention relates to an isolated human mega¬ karyocyte-colony stimulating factor protein (hMeg-CSF), to pharmaceutical formulations comprising said factor, and to methods relative to producing, isolating and using the same. BACKGROUND OF THE INVENTION In humans, the hematopoietic i.e., the blood-forming system, includes bone marrow and the blood. The bone marrow is responsible for producing the cellular elements of the blood. Mammalian (including human) blood is composed of minute cellu¬ lar fragments which are called platelets or thrombocytes, and highly specialized cells called red blood cells (erythrocytes) and white blood cells (leukocytes) all of which are suspended in the plasma. Platelets perform an essential function in the mammalian clotting system: in response to wound-induced chemi¬ cals, they cause blood to clot. The red blood cells are responsible for imparting to blood its characteristic deep red color and for transporting oxygen (0 2 ) and other nutrients to the tissues throughout the mammalian body where the 0 2 and other nutrients are exchanged for carbon dioxide (C0 2 ) and waste products. The white blood cells on the other hand are responsible for defending the body against infection. Because the blood in the body is in constant motion, i.e., it is circulating through a closed network of blood vessels, both platelets and blood cells are normally well-suspended in the plasma. The mature red and white blood cells as well as the platelets, i.e., the cellular components of the blood, are formed from primitive undifferentiated precursor cells produced in the mammalian bone marrow. These undifferentiated precursor cells are variously referred to as pluripotent stem cells or progenitor cells. Each stem cell has the potential to dif¬ ferentiate and develop into either mature erythrocytes (red blood cells), or leukocytes (white blood cells) or megakaryocytes (producers of platelets). For this reason, the stem cells are regarded as primitive pluripotent precursors to the mature erythrocytes, leukocytes and megakaryocytes. In other words, the highly specialized blood cells of the hematopoietic system are developed from the primitive undif- ferentiated stem cells produced in the bone marrow, as il¬ lustrated in Fig. 6. It is generally accepted today that stem cell growth and differentiation into mature blood cells, i.e., either erythrocytes, leukocytes or megakaryocytes, is regulated by ap- propriate hematopoietins. Hematopoietins, which are also commonly known as blood-cell growth factors, are a specialized group of glycoproteins that promote growth and differentiation of pluripotent stem cells into mature blood cells. See Fig. 6. Colony stimulating factors (CSFs) are a specific class of hematopoietic growth factors or proteins which are believed to be capable of initiating precursor cell proliferation and differentiation into the different types of mature blood cells. In other words, CSFs are believed to be responsible for causing primitive undifferentiated precursor cells to commit to, and develop in accordance with, a specific blood cell lineage, i.e., to commit to either the erythrocytic, leukocytic or megakaryocytic lineage. Thus, the particular type of mature blood cell that results from a primitive undifferentiated precursor cell depends upon the type of hematopoietins the stem cell encounters. For instance, erythropoietin (EPO) causes primitive undifferentiated precursor cells in the bone marrow to commit to the erythrocytic lineage, i.e., to differentiate and mature into erythrocytes, whereas granulocyte/macrophage colony-stimulating factor (GM-CSF) is believed to cause the precursor cells to differentiate and mature into particular types of leukocytes called granulocytes and monocytes. Platelets are terminal products of megakaryocyte differentiation. Megakaryocytes also originate from primitive undifferentiated precursor cells of the bone marrow, as illus- trated in Fig. 6. The earliest recognizable member of the megakaryocyte series developed from stem cells in the bone marrow is the megakaryoblast, which possesses an immature nucleus embedded in a basophilic cytoplasm with a minimum number of granules. Megakaryoblasts and megakaryocytes can be identified by specific cell surface markers; acetylcholinester- ase in murine cells and Ilb/IIIb in human cells. Through a complex maturation process, megakaryoblasts mature into megakaryocytes; the process involves the formation of a ulti- lobulated polyploid nucleus and distinctive, highly specialized cytoplasmic granules, as illustrated in Fig. 6. The mature megakaryocytes form platelets by pinching off fragments of their cytoplasm and releasing them into the circulating blood, as depicted in Fig. 6, by a process that is not yet well-under¬ stood. As indicated above, platelets are critical minute cytoplasmic particles which regulate blood clotting. Depletion of the circulating level of platelets, called thrombocytopenia, occurs in various clinical conditions and disorders. Thrombo¬ cytopenia is dangerous because patients with this condition are subject to uncontrolled bleeding episodes. If the cause of thrombocytopenia is an external insult or injury (as opposed to a disorder in the production, or maturation of megakaryocytes and platelets) platelet levels are usually restored within a short period of time (about 4-5 days in humans) if the injury or insult (chemical) has been removed. But if a platelet disorder underlies this condition, it persists for as long as the disorder is present, often throughout the patient's life. The only treatment up to the present time has been frequent platelet transfusions with all the attendant dangers that it entails (which range from infection to immune reaction) . Approximately 240,000 patients in the United States are under¬ going chemotherapy and are thrombocytopenic. In addition 50,000 patients with other diseases that are not cancer-related are thrombocytopenic. Accordingly, there is an acute need in the art of identifying means and methods to promote platelet production in humans. Although progress has been made in identifying the megakaryocyte-platelet specific hematopoietins, relatively little is known about the regulation of megakaryocytopoiesis, i.e., megakaryocyte production. Several humoral factors have been postulated to control the maturation of megakaryocytes. One substance recently obtained is termed thrombopoietin (TPO) or thrombopoietic stimulatory factor (TSF), depending upon the source from which the activity is derived. Recently, evidence has been accumulating that there is a dual level of regulation of megakaryocytopoiesis, involving more than one regulatory factor. Recent data suggest that a megakaryocyte colony- stimulating factor (Meg-CSF) is involved in the first phase of megakaryocytopoiesis. Meg-CSF appears to be responsible for causing primitive undifferentiated precursor cells in the bone marrow to commit and differentiate into egakaryocytic lineage type cells. Recent data also indicate that the second phase of megakaryocytopoiesis i.e., the maturation of the committed precursor cells to fully differentiated and mature megakaryocytes, is regulated by thrombopoietin and that in fact the blood concentration of thrombopoietin is in turn influenced by changes in the level of circulating platelets. See, for example. Murphy, M.J., et al.. Acta Haematol. JPN. 46(7): 1380-1396 (1983); Hoffman, R. , et al., J. Clin. Invest. 25:1174-1182 (1985); Kuriya, S., et al. , Blood Cells 12:233-247 (1986); Yang, Y-C, et al., J. Clin. Invest. 77:1873-1880 (1986); Kuriya, S., et al ' . , EXPI. Cell. Biol. 55:257-264 (1987); Hirano, T., Int. J. Cell Cloning 8 (Suppl. 1):155-167 (1989); Hoffman, R. et al., Hematol. /Oncol. Clinics of N. Amer. Hematol. Oncol. 3:467-478 (1989); McDonald, T.P., Int. J. Cell Cloning 7:139-155 (1989); Murphy, M.J., Hematol/Oncol. Clinics of N. Amer. Hematol. Oncol. 3(3):465-478 (1989); and Ogata, K., et al.. Int. J. Cell Cloning 1:103-120 (1990). Previous attempts have been made in the past to identify and isolate various hematopoietins including a Meg- CSF. For example, Teramura, M. , et al., Exp. Hematol. 16 :843- 848 (1988), disclose an interleukin-3 (IL-3) protein. This protein has a molecular weight of about 14-28,000 daltons and an isoelectric point (Pi) equal to about 4.5-8.0. Moreover, while it has been reported that this IL-3 protein has the ability to stimulate formation of granulocyte, macrophage, erythroid, and megakaryocyte colonies from bone marrow in vitro, the human form of the IL-3 protein is species-specific and unable to produce megakaryocyte colonies in an in vitro murine fibrin clot assay. Williams, N., et al., Exp. Hematol. 12:734-740 (1984), report the action of erythropoietin (EPO). EPO is an acidic protein having two peaks with a Pi equal to about 3.1- 3.5 and 4.4-4.9, respectively, and a molecular weight of about 34-39,000 daltons. In addition EPO is credited with the ability to stimulate formation of erythroid and megakaryocytic colonies from bone marrow in vitro. See also, Kawakita, M. , et al.. Human Urinary Megakaryocyte Colony -and Thrombopoiesis - Stimulating Factor, in: Megakaryocyte Development and Function, 201-208 (1986); Ishibashi, T., et al., J. Clin. Invest. 79:286- 289 (1987); and Sakaguchi, M. , et al., Exp. Hematol. 15:1028- 1034 (1987). However, EPO does not stimulate platelet produc¬ tion in vivo. Mazur, E.M., et al., Exp. Hematol. 15:1128-1133 (1987), report the isolation of a granulocyte/macrophage colony-stimulating factor (GM-CSF) protein. GM-CSF is reported as being an acidic protein having a molecular weight of about 14-35,000 daltons and a Pi equal to about 4.5-5.3 (when derived from serum-free conditions) or about 4.0-4.6 (when derived from serum-containing conditions). GM-CSF is reported to stimulate formation of granulocyte-macrophage and megakaryocytic colonies from bone marrow in vitro but it is species-specific. See also, Ishibashi, T. , et al., Blood 75:1433-1438 (1990). An interleukin-6 (IL-6) protein has also been report- ed in the literature. IL-6 has a molecular weight of about 21- 26,000 daltons and a Pi equal to approximately 6.2-6.4 as determined by chro atofocusing of plasmacytoma growth factor. Although the IL-6 is reported as having the ability to increase mature megakaryocyte size, the number of megakaryocytes with higher ploidy, and the number of cells in a megakaryocyte colony (all which are maturation-type functions) in vivo, the reported IL-6 lacks the ability to produce megakaryocyte colonies in vitro. See, for example, Lotem J., et al.. Blood 14:1545-1551 (1989); Ishibashi T., et al., Proc. Natl. Acad. Sci. .86 . 55953-5957 (1989); and Bruno E., et al., Exp. Hematol. 11:1038-1043 (1989). Ishibashi T. , et al., Proc. Natl. Acad. Sci. 86:5953- 5957 (1989), report an isolated thrombopoietin (TPO). The TPO protein is said to be an acidic protein having a molecular weight of about 15,000 daltons and a Pi equal to about 4.5. In addition, the thrombopoietin protein is stable to 2-mercap- toethanol. This TPO is reported to increase the diameter of megakaryocytes, a maturation-type function, but is unable to stimulate formation of megakaryocytic colonies from bone marrow in vitro. See also, McDonald, T.P., et al.. Int. J. Cell Cloning 2:139-135 (1989); Williams, N., et al., Exp. Hematol. 12:734-740 (1984); Williams, N., et al.. Blood Cells 5:43 (1979); Levin J. , et al.. Blood 6J):989 (1982); and Straneva, J.E., et al., Exp. Hematol. 17:1122-1127 (1989). Rosenberg, R.D. discloses a megakaryocyte stimulating factor (MSF) protein in his U.S. Patent No. 4,894,440. Rosen¬ berg reports further that this MSF is an acidic protein having a Pi equal to 5.1 with a molecular weight of approximately 15,000 daltons but does not exhibit Meg-CSF activity (U.S. Patent No. 4,894,440, col. 3, lines 65-68.) In other words, even though Rosenberg's MSF is said to increase synthesis of platelet factor 4 (PF4) and the rate of megakaryocytic cytoplasmic maturation, it is unable to stimulate formation of megakaryocytic colonies from bone marrow in vitro. See also Greenberg, S.M., et al., J^., Biol. Chem. 262:3269-3277 (1987); and Tayrien, G., et al., J. Biol. Chem. 262:3262-3268 (1987). Yang, Y-C, et al., Bj-ood 14:1880-1884 (1989), report an interleukin-9 (IL-9) protein. IL-9 is reported to have a molecular weight of approximately 20-30,000 daltons or a molecular weight of 32-39,000 daltons based on information for murine p40, the murine homologue of human IL-9, and a Pi equal to approximately 10 based on flow-through on MonoQ Chromatog- raphy at pH 9.5, as reported by Uyttenhove, C, et al., Proc. Natl. Acad. Sci. 85:6934-6938 (1988). The IL-9 protein is reported as having the ability to stimulate erythroid colony formation in vitro: see Yang, Y-C, et al.. Blood 74 (Suppl. 1) :116a (1989); but no ability to stimulate formation of megakaryocyte colonies in vitro. Donahue, R.E. et al.. Blood. 1990, 15:2271-2275. Williams, N., et al.. Studies on Megakaryocyte Poten¬ tiator: Its Production and Some Biochemical Characteristics. In: Megakaryocyte Development and Function, pp. 91-103 (1986), report a Meg Potentiator protein having a molecular weight of approximately 21,000 daltons and three peaks having a Pi equal to about 4.0, 5.0 and 6.0, respectively. The Meg Potentiator protein is reported to increase ploidy of megakaryocytes, a maturation type function, but it did not stimulate formation of megakaryocytic colonies in vitro. See also. Sparrow, R.L., et al., Leukemia Res. 11:31-36 (1987). In addition to the above, investigators have reported alleged Meg-CSF type proteins. For instance, Kawakita, M. , et al., Br. J. Haematol. 52:429-438 (1982), reports a protein having a molecular weight of approximately 155,000 daltons and 76,000 daltons by gel filtration (Sephadex G-200), and 45,000 daltons when the gel filtration is performed with 6 M guanidine. Kawakita, M. , et al.. Human Urinary Megakaryocyte Colony - and Thrombopoiesis - Stimulating Factor, in: Mega¬ karyocyte Development and Function, pp. 201-208 (1986), report the presence and biological properties of alleged Meg-CSF and TSF type proteins from urine of aplastic anemia patients. Therein, Kawakita indicates that the alleged Meg-CSF type protein after IEF assay by plasma clot cultures has two dis¬ tinct peaks wherein the first peak elutes at a Pi of between 3.1-3.5 and the second peak elutes at a Pi of 4.4 with a shoulder at 4.7-4.9. This data suggest that this alleged Meg- CSF type protein material includes asialo-EPO. Hoffman, R. , et al., J. Clin. Invest. 75:1174-1182 (1985) report an alleged homogeneous Meg-CSF type protein having a molecular weight of 46,000 daltons as determined by SDS-PAGE. In a later publica- tion (Hoffman, R., Blood 74 . :1196-1182 (1985)) however, they report that this protein material was not at a level of purity which would allow for accurate amino acid sequencing. Ogata, K., et al., Exp. Cell. Biol., 5_7:1926 (1989), report a partial¬ ly purified protein that is substantially contaminated with EPO. Still further, Mazur, E.M., et al., Exp. Hematol. 13:1164-1172 (1985), report a partially purified protein from canine material having a molecular weight of approximately 175,000 daltons as determined from Sephacryl S-300. Mazur, E.M. et al. report therein that the protein material is inac- tivated by trypsin, 5 mM DTT, 6 M guanidine and 8 M urea. The present inventors also have reported the alleged existence of a Meg-CSF protein, and have likewise made unsuc¬ cessful attempts in the past to confirm the existence and to isolate and/or characterize a pure homogeneous human Meg-CSF type protein. See Kiyoyuki, 0., et al.. Int. J. Cell Cloning 1:103-120 (1990); Murphy, M.J., Hematol. /Oncol. Clinics N. Amer. 3(3):465-478 (1989); Kuriya, S-I., et al., Exp. Hematol. 1^:896-901 (1987); Kuriya, S-I., et al., Expl. Cell Biol. 55.-257-264 (1987); Murphy, M.J., et al., Acta Haematol. Jpn. , ϋ:1380-1396 (1983); and Miyake, T. , et al., Stem Cells 2:129- 144 (1982). PCT application WO 91/02001 published February 21, 1991 purports to be directed to the purification of an alleged human megakaryocytopoietic factor isolated from bone marrow transplant patients and said to be capable of stimulating megakaryocyte colonies by the murine fibrin clot and agar Meg- CSF assays. The molecular weight of this isolate by SDS-PAGE (12%) is said to be within the range of 20-27 kD under reducing and 28-38 Kd under non-reducing conditions. Although this protein is said to be "homogeneous" there is no guideline as to what this term means. More important, there is no demonstra¬ tion that this "factor" is indeed free of other cytokines, such as EPO and GM-CSF, which are known to have activity similar to hMeg-CSF. Geno ic DNA sequences (and the predicted corre- sponding amino acid sequences) are also disclosed in WO 91/02001 and speculated to contain somewhere within them sequences encoding the foregoing factor. These DNA sequences were isolated based on DNA probes derived from tryptic frag¬ ments of the alleged Meg-CSF factor isolated according to the WO 91/02001 procedures. But no identification of e.g. an N- ter inal has been made and no sequence is disclosed that is confirmed to encode a polypeptide having Meg-CSF activity. Additionally, the purification scheme of WO 91/02001 contains different steps and conditions from the scheme disclosed below. Thus, although numerous investigators have attempted to locate and isolate a Meg-CSF protein, the existence, iden¬ tity, structure and biological activity of this postulated Meg- CSF protein (or proteins) has up to now remained elusive and controversial. Consequently, there are serious needs in the scientific and medical communities to confirm the existence, identity and activity of a human Meg-CSF protein and to iso¬ late, sequence, and reproduce same for purposes of, among other things, combatting and better understanding the causes of thrombocytopenia, and the mechanism of platelet production. SUMMARY OF THE INVENTION The present invention alleviates the above-mentioned problems and shortcomings of the present state of the art through the discovery of a novel, isolated homogeneous human megakaryocyte-colony stimulating factor (hMeg-CSF) and methods of obtaining same. In one aspect, the present invention is directed to an isolated, purified human megakaryocyte colony stimulating factor, said factor having the following properties: a) being free of detectable EPO and GM-CSF activi¬ ties; b) being homogeneous as determined by existence of a single amino terminal amino acid sequence and migration as a single band after electrophoresis on sodium dodecyl sulfate polyacrylamide gels; and c) having the ability to induce the formation of megakaryocyte colony-forming units in a murine fibrin clot assay in vitro, with and without the addition of serum. The single N-terminal amino acid sequence has been partially identified and the molecular weight of the species with this N-terminal sequence has been typically found to be 52-55 kD with activity within the range of 50-70 kD. Another smaller species with the same activity has a molecular weight of 24-35 kD and is often co-present in the homogeneous prepara- tions of the larger species, especially under reducing condition In another aspect, the present invention is directed to an isolated, purified human megakaryocyte colony stimulating factor preparation comprising at least about 90% protein said preparation being characterized as: a) being free of EPO, GM-CSF, IL-3, IL-9, IL-6 and all other cloned cytokine activities, b) having the ability to induce the formation of megakaryocyte colony forming units in a murine fibrin clot assay in vitro. This highly purified but non-homogeneous hMeg- CSF fraction can be used to elucidate the sequence of hMeg-CSF as well as the mechanism of platelet production. The molecular weight of human Meg-CSF protein species in this virtually pure fraction, when the protein is in gly¬ cosylated and sialyated form is within the range of about 24,000 daltons and about 35,000 daltons for the smaller species and between about 50,000 daltons and 70,000 daltons for the larger species, both as determined by SDS-PAGE. Its iso- electric point is within the range of between about 7.2 and 7.4 as determined by isoelectric focusing for both species. The hMeg-CSF protein both in homogeneous and in highly purified (virtually pure) fraction form has the characteristics identi¬ fied for hMeg-CSF and used to distinguish hMeg-CSF from other hematopoietic proteins. A further aspect of the present invention is directed to isolated, purified recombinant polypeptides having human megakaryocyte colony stimulating factor activity and to methods for isolating DNA encoding this factor. Another aspect of the present invention is directed to a pharmaceutical formulation for administration to a mammal suffering from a disease related to the production of platelets comprising an isolated, purified human megakaryocyte colony stimulating factor protein, said protein having the following properties: a) being free of EPO and GM-CSF activities, b) being homogeneous as determined by having a single amino terminal amino acid sequence and migrating as a single band after electrophoresis in sodium dodecyl εulfate polyacrylamide gels; and c) having the ability to stimulate the formation of megakaryocyte colony forming units in a murine fibrin clot assay in vitro. A still further aspect of the present invention is directed to a pharmaceutical formulation for administration to a mammal suffering from a disease related to the production of platelets comprising an isolated, purified polypeptide having human megakaryocyte colony stimulating factor activity and comprising at its amino terminus the amino acid sequence X-Asp- Pro-Val-Glu-Ser-Pro-Val-Pro-Y (wherein X and Y are unspecified amino acid residues) . It should be noted that the smaller species can be used in the foregoing formulation instead of or in addition to the larger species. Yet another aspect of the present invention is directed to a method for isolating a human Meg-CSF protein fraction said fraction having a protein content of at least 90% and being free of EPO and GM-CSF activity, said method compris¬ ing the steps of: a) concentrating urine from patients having Meg-CSF activity in their urine (e.g., aplastic anemia patients); b) desalting the concentrated urine; c) removing non-ionic contaminants contained in the desalted concentrated urine by applying it to an ion exchange support and eluting from said support an impure protein frac- tion containing human Meg-CSF; d) applying the impure protein fraction to a preparative polyacrylamide electrophoresis gel under non- denaturing conditions and isolating from said gel a substan¬ tially pure Meg-CSF fraction; e) subjecting said substantially pure Meg-CSF fraction to a further purification step selected from the group consisting of i) chromatofocusing chromatography using a gel substituted with tertiary and quaternary amines; ii) ion-exchange chromatography using a cation- exchange high performance liquid chromatog¬ raphy column; and iii) gel electrofocusing at a pH gradient be¬ tween about 3.5 and about 10 and recovering a further purified Meg-CSF fraction; f) subjecting said further purified fraction to reverse phase high performance liquid chromatography and recovering a hMeg-CSF fraction containing at least 90% protein and being free of EPO and GM-CSF activity. A variation of the foregoing purification scheme comprising chromatofocusing in step (e) and further comprising step (g) cation exchange HPLC following step (f) yields homogeneous human Meg-CSF (which includes either the larger species alone or a combination of the larger and the smaller species) . These and other aspects of the present invention will be apparent to those of ordinary skill in the art in light of the present description, claims and drawings. BRIEF DESCRIPTION OF THE FIGURES Figs. 1A and IB are general outlines of the preferred steps used in the present invention to isolate hMeg-CSF protein in substantially pure, virtually pure, functionally homogeneous and homogeneous form, from urine of aplastic anemia patients. Fig. 2 is a graphic illustration of various hMeg-CSF containing fractions recovered following preparative 5% polyacrylamide gel electrophoresis of crude urine extract obtained from aplastic anemia patients in accordance with the method of the present invention (specifically Fig. 1A) . Fig. 2 also illustrates graphically the number of CFU-Meg and CFU-GM colonies stimulated by the protein fractions. Substantially pure hMeg-CSF protein is in fractions 1-5. Fig. 3 is a graphic illustration of various protein fractions isolated from an isoelectric gel run at a pH range of about 3.5-10 in accordance with the methods of Fig. 1A using the DEAE-Cellulose ion exchange support and the IEF pathways. Fig. 3 also illustrates graphically the amount of protein per fraction, the number of CFU-Meg colonies stimulated by each protein fraction, and the pH of each protein fraction. Virtu¬ ally pure hMeg-CSF protein is in fraction number 16. Protein fraction number 16 stimulates the formation of the largest number of CFU-Meg colonies in vitro (compared to the other fractions, as illustrated in Fig. 3) and has a pH of between about 7.2-7.4. Fig. 4 is a graphic illustration showing the absor- bance at 280 nm, the number of CFU-Meg colonies formed and the pH of groups isolated from the CM-Sepharose ion exchange support and the MonoP chromatofocusing column pathways in accordance with the methods of Fig. 1A. Virtually pure hMeg- CSF protein is in group number 5. Protein group number 5 stimulates the formation of the largest number of CFU-Meg colonies and has a pH of about 7.0-7.5. The protein fraction under group number 5 has three main peaks of protein. Fig. 5 is a graphic illustration showing the number of CFU-Meg colonies formed, the percent of solvent (acetonitri- le) and the absorbance at 280 nm of protein fractions obtained from the CM-Sepharose ion exchange support, the MonoP and the C18 reverse-phase HPLC pathways in accordance with the methods of the instant invention (Fig. 1A) . Isolated, hMeg-CSF protein is in fraction numbers 32-34. Protein fraction numbers 32-34 stimulate the formation of the largest number of CFU-Meg colonies and fall between two absorbance peaks as illustrated. Fig. 6 is a pictorial illustration generally depict¬ ing the development and differentiation of the various compo¬ nents of the blood from a primitive precursor undifferentiated pluripotent stem cell in accordance with the erythrocytic, leukocytic and megakaryocytic lineages. Fig. 6 has been generally quoted and reproduced from a photograph appearing in the Schering Plough/Sandoz Pharmaceuticals 1990 calendar for background purposes and, more particularly, for illustrating and generally teaching the development, differentiation and production of the highly specialized blood cells and platelets from stem cells in the bone marrow. Fig. 7 is a photograph of a silver stained SDS-PAGE gel of a purified Meg-CSF-containing preparation of the inven¬ tion and depicts in vertical lane labelled "fraction number 32- 34", protein bands of isolated functionally homogeneous hMeg- CSF species, one having a molecular weight of about 24,000- 35,000 daltons and the other having a molecular weight of about 50,000-70,000 daltons, as determined by 12% analytical SDS-PAGE stained with silver (BioRad, Richmond, CA) . The five vertical lanes depicted in this Fig. 7 contain fractions from C18 reverse-phase HPLC generated in accordance with the method of Fig. 1A. Molecular weight markers (kD) are shown to the right of the five fraction lanes. The hMeg-CSF protein of this Fig. 7 is isolated via the CM-Sepharose ion exchange support, the MonoP and the C18 reverse phase HPLC pathways in accordance with the methods of this invention. Figure 8 depicts the results of Meg-CSF transfer to Immobilon PVDF Membrane and in particular the single band obtained upon SDS-PAGE of homogeneous hMeg-CSF (produced according to the Fig. IB scheme) and transfer to an Immobilon PVDF membrane. This single band is the higher .w. species. The lower m.w. species is not evident. Fig. 9 is a chromatographic profile of Meg-CSF purification on a polyaspartic acid (WCX) HPLC column. Absor- bance was monitored at 280 nm with a full scale of 0.1 absor- bance units. Flow rate is 1 mL/min. Initial column equilibra¬ tion is in 0.05 M sodium phosphate, pH 6.0. The gradient of increasing NaCl is signified by the dashed line. 2.5 ml frac¬ tions are collected and pooled as follows: pool fraction A 1-6 B 7-11 C 12-15 D 16-21 E 22-28 CFU-Meg activity for each pool is reported at the bottom. Fig. 10A is a photograph of a silver stained SDS-PAGE gel of individual fractions #16-28 from two WCX HPLC columns concentrated by Centricon 10 (A icon: Danvers, MA). Aliquots of individual fractions were put in SDS-PAGE sample buffer (non-reducing conditions) and run on 12% analytical SDS-PAGE. Molecular weight markers are as follows: 110,000, 84,000, 47,000, 33,000, 24,000, 17,000 as shown on the right of the gel. Only the 50-70 kD species is present as a single band migrating with a molecular weight of about 52 kD. Fig. 10B is a bar diagram of the Meg-CSF activity of other aliquots of the individual fractions 16-30 from the two HPLC WCX columns above which were sterile filtered and assayed for biological activity by murine fibrin clot assay. Fig. 11A is a bar diagram showing the Meg-CSF activi¬ ty profile of SDS-PAGE elution as a function of gel slice position. Pools D + E (fractions No.16-28) from WCS cation exchange HPLC, were run into 12% SDS-PAGE, the gel lane was sliced, and each slice was eluted into IMDM + 10% FCS (1 mL/cm gel), dialyzed for 2 days against distilled water, sterile- filtered and assayed for biological activity. This figure reports the biological activity of each slice and its corre¬ sponding molecular weight after SDS PAGE under non-reducing conditions. Fig. 11B is the same type of diagram generated after SDS PAGE under reducing conditions. A shift in activity from the high m.w. species to the low m.w. species can be seen, which indicates that the 24-35 kD species may be a fragment or- monomer of the 50-70 kD species. Fig. 12 is an autoradiograph of DNA produced by polymerase chain reaction. Placenta genomic DNA was purchased from Clontech Lab, Inc. (CA) . One μg of the DNA was amplified using oligo 1 and 3 according to the protocol provided by Perkin-Elmer Cetus Inc. The amplified products (1.65 kbp and 300 bp) were eluted with Geneclean (Bio 101, Inc. CA) and 10 μg were reamplified by PCR. The PCR products were analyzed by 1% agarose gel electrophoresis. Lanes 1 to 3 represent 300 bp DNA amplified with the presence of oligo 2 alone, oligo 3 alone, oligo 2 and 3, respectively. Lanes 4 to 6 represent 1.7 kbp DNA amplified with oligo 2 alone, oligo 3 alone, oligos 2 and 3 with two Mg ++ concentration, respectively. Lane 7 contains size markers. Lanes 9 and 10 show the 300 bp and 1.7 kbp fragments re-amplified respectively with oligos 1 and 3. Lane 11 represents the primary amplification product of genomic DNA using oligos 1 and 3. Lane 8, molecular marker (one kb ladder, BRL) . DETAILED DESCRIPTION OF THE INVENTION All literature references, patent applications and patents referred to in this specification are hereby incor- porated by reference in their entirety. "Mammals" is defined herein to mean any organism having a hematopoietic system and susceptible to a disease related to the production of platelets and includes humans. "Functionally homogeneous human Meg-CSF" is defined herein as a hMeg-CSF fraction, although not purified to homoge¬ neity, contains no other detectable hematopoietins (as assessed by standard activity tests: Absence of erythropoietin contami¬ nation is assessed by murine spleen cell assay according to Krystal G., Exp. Hematol., 1983, 11:649-660, which can detect as little as 0.05 units/ml of EPO activity; absence of GM-CSF, IL-3 and IL-9 is assayed by M-07-e bioassay according to Avanzie, G.C. et al. J. Cell Physiol. , 1990, 145:458-464 which has a sensitivity limit of 12.5 units/ml GM-CSF 6 units/ml for IL-3, and 5 units/ml for IL-9; absence of IL-6 and other cytokines is assessed by ELISA by Quantikine™ (R & D Systems, Minneapolis, MN) which can detect as little as 6 pg/ml IL-6 and 31.3 pg/ml of IL-1 alpha and comparable amounts for other cyto¬ kines. A functionally homogeneous fraction according to the scheme of Fig. 1A is sequenceable, i.e., an hMeg-CSF protein contained in this fraction was used to generate the amino terminal amino acid sequence obtained as shown in Example 5 below, which was identified from the larger species (50-70 kD) even though the smaller species (24-35 kD) was also present in the preparation. Either the smaller species does not have the amino terminal or it is blocked (i.e. not free) or the amount of material available was not sufficient to permit sequencing of the N-terminal from the smaller species. (It should be noted that the preparation of Fig. 7 which is "functionally homogeneous" does contain other bands in addition to the two species having hMeg-CSF activity. Therefore, there is no implication here that only the two hMeg-CSF species may be present in a functionally homogeneous preparation. ) "Homogeneous human Meg-CSF" is defined herein as a polypeptide which has human Meg-CSF activity, migrates as a single band upon electrophoresis in SDS-PAGE gels and after transfer to PVDF Immobilon membrane has a single amino terminal amino acid sequence. While this definition is cast in terms of the higher m.w. species, copresence of the lower m.w. species should not be interpreted as negating homogeneity. The 24-35 kD species has hMeg-CSF activity of its own. This is apparent in Fig. 11B. Moreover, the fact that a preparation containing both species is still "functionally homogeneous" and does not have any other cytokine negates the possibility that the lower m.w. species is a contaminant. At this point, the relationship between the two species has not been conclusively established. It is not known for example whether the smaller species is a fragment or a monomer of the larger species (as Figs. 11A and 11B suggest), or simply another protein altogether which happens to have hMeg-CSF activity. Human Meg-CSF protein (of either or both molecular weights) can be isolated from the urine of patients suffering from aplastic anemia (or another condition, such as bone marrow transplant or thrombocytopenia of a different origin which causes Meg-CSF activity to be present in the urine), but should be purified to homogeneity for therapeutic use. hMeg-CSF derived from natural sources should also be purified to func¬ tional homogeneity or homogeneity before it is sequenced or otherwise used in preparation of recombinant or synthetic techniques for producing recombinant hMeg-CSF (e.g., for producing monoclonal antibodies). Because of species cross- reactivity between mouse and human, it is anticipated that the hMeg-CSF of the present invention will be useful in treating other mammals, such as pets which are in need of such treatment (e.g., pets undergoing chemotherapy). Both species of Human Meg-CSF protein are weakly basic protein species and are believed to be specific for stimulating the proliferation of megakaryocytic lineage type cells and platelet production in vivo. The hMeg-CSF protein of the instant invention has a Pi of about 7.2-7.4 as determined by isoelectric-focusing and a molecular weight ranging between about 50,000 and 70,000 daltons (or 24,000 and about 35,000 daltons for the smaller species) as determined by SDS-PAGE when the hMeg-CSF protein is in the glycosylated and sialyated form, as illustrated in Fig. 7. It is believed that the carbohydrate residues including the biantennary carbohydrate structures can be cleaved from the hMeg-CSF protein (either species) via ap¬ propriate glycosidases, such as endoglycosidase F, en- doglycosidase H and N-glycanase. The sialic acid moieties of the hMeg-CSF protein can be removed by treatment with neuramin- idase. Even when the carbohydrate and sialic acid moieties are cleaved from the hMeg-CSF protein of the instant invention (both species) to form a naked hMeg-CSF protein, it retains its biological activity in vitro. The novel hMeg-CSF protein of the instant invention, which is preferably purified to homogeneity, has the ability to regulate megakaryocytopoiesis and platelet production. More particularly, the novel hMeg-CSF protein of the present inven¬ tion has the ability to stimulate proliferation of megakaryo¬ cytes and production of platelets in vivo, and has the further ability to stimulate proliferation of megakaryocytic lineage type cells, e.g., megakaryocyte-colony forming units into megakaryoblasts, in an in vitro murine mouse megakaryocyte- colony forming fibrin clot assay as well as in a serum-free system. (See, e.g. , Murphy, M.J. et al., J. Tiss Cult. Meth. , 1991, 13:83-88.) In other words, each species is able to induce primitive precursor cells in the bone marrow to commit to, and to grow and differentiate in accordance with, the megakaryocytic lineage. In scientific terms, the present invention, which has eluded the scientific and medical com- munities heretofore, is predicated upon the confirmed discovery (as well as the isolation, purification and characterization) of a novel human hematopoietin or blood cell growth factor, i.e., a human megakaryocyte colony-stimulating factor, which is believed to be involved in at least the first phase of human megakaryocytopoiesis. In layman's terms, the present invention is based upon the discovery of a unique protein produced by humans which is specifically involved in the production of blood platelets. The higher m.w. hMeg-CSF protein of the present invention is a weakly basic, homogeneous protein as judged by isoelectric focusing, SDS-PAGE, chromatofocusing, C18 reverse- phase and WCX cation exchange HPLC and is characterized as having the single N-terminal amino acid sequence X-Asp-Pro-Val- Glu-Ser-Pro-Val-Pro-Y, wherein X and Y represent as yet unde- termined amino acid residues. While the novel hMeg-CSF protein retains its ability to increase platelet counts and bone marrow megakaryocyte numbers vivo following amidation or treatment with neura inidase, it has been found to lose some activity following treatment with 5,5,-dithio bis 2-nitrobenzoic acid (DTNB). Reduction of the hMeg-CSF of the invention does not by itself cause inactivation although it causes a shift of the bulk of the activity towards the smaller species. However, reduction (by dithiothreitol) followed by alkylation (with iodoacetamide) or mercuration (with mercury chloride) causes deactivation of the smaller species which is the predominant species after reduction. In addition, the biological activity of the novel hMeg-CSF protein of the instant invention is believed to be lost when greater than about 30% of the amino acid residues thereof are carbamylated (since carbamylation was not preceded by reduction both species are presumed to have been inactivated). These findings are consistent with pub¬ lished characteristics of impure hMeg-CSF and serve to confirm that the isolated purified activity of the present invention is the same as that present in impure preparations of the prior art having hMeg-CSF activity and is not the same as the activi¬ ty of other reported hematopoietins. The hMeg-CSF protein of the instant invention is further characterized as a glycoprotein having biantennary carbohydrate structures and beta-galactose residues as the terminal or penultimate sugars. The novel hMeg-CSF of the instant invention is further characterized as containing sialic acid. These characteristics are confirmed by the binding characteristics of the hMeg-CSF protein to an RCA I agarose. Con A Sepharose and Lentil Lectin columns in sialyated and desialyated forms. In sialyated form, it has been found that approximately 16% hMeg-CSF protein activity binds to an RCA I agarose column, about 5% hMeg-CSF protein activity binds to a Con A Sepharose column, and approximately 28% hMeg-CSF protein activity binds to a Lentil Lectin column. In the desialyated form, however, it has been found that approximately 56% hMeg- CSF protein activity binds to an RCA I agarose column, about 41% hMeg-CSF protein activity binds to a Con A Sepharose column (about 31% activity following elution with about 15 mM alpha- methylglucoside or about 10% activity following elution with about 200 mM alpha-methylglucoside) , and about 45% hMeg-CSF protein activity binds to a Lentil Lectin column. The portion of the binding activity that is attributable to each species has not been determined. Two different preferred methods for isolating hMeg- CSF are described hereinbelow. Both procedures have identical first phases up to the preparative PAGE step. In one embodi¬ ment, "functionally homogeneous" hMeg-CSF is produced (See Fig. 1A) , whereas using an alternative embodiment, pure "homoge¬ neous" hMeg-CSF is produced (See Fig. IB). Both isolation methods provide useful materials as both the "functionally homogeneous" and the pure "homogeneous" hMeg-CSF fractions (even those homogeneous fractions containing both or either species) can be used for amino acid sequencing and administra¬ tion to mammals as further described below. Also, "virtually pure" hMeg-CSF (i.e., the product of scheme lA but without the last step) is believed to have the same uses. The present invention also contemplates novel methods for isolating the hMeg-CSF protein(s) of the instant invention. In one embodiment, hMeg-CSF is obtained, preferably in func- tionally homogeneous form, from urine of thrombocytopenic patients. Preferred procedures for isolating hMeg-CSF in functionally homogeneous form are generally outlined in Fig. 1A and are typically performed in two phases. In the first phase a protein fraction containing hMeg-CSF but contaminated with EPO and GM-CSF proteins is produced. In the second phase functionally homogeneous hMeg-CSF invention is produced. Prior to obtaining the isolated functionally homogeneous hMeg-CSF protein, however, a substantially pure hMeg-CSF protein frac¬ tion is produced following the preparative PAGE step of the second phase. The term "substantially pure hMeg-CSF protein fraction" is used herein to refer to a protein fraction which is believed to comprise at least about 50% of hMeg-CSF protein and to be essentially free of contaminating EPO and GM-CSF proteins. "Essentially free of contaminating EPO and GM-CSF" is defined herein based on a content of less than 100 units GM- CSF per mg protein and less than 0.5 units EPO per mg protein as determined e.g. , by the Krystal assay (Krystal, G. Exp. Hematol. 1983, 11:649-660) using murine thymocytes (for EPO) and by murine CFU-GM assay (for GM-CSF) according to Du, D-L, et al. Invest. New Drugs, 1991, 9:149-157. However, other cytokines may still be present in "essentially free" prepara¬ tions: for example, the material of Example 1, step D does contain approximately 1-2x10* units M-CSF and G-CSF per mg protein. In other words, it is believed that the preparative PAGE step of the second phase of the methodology is responsible for eliminating from the generated substantially pure hMeg-CSF protein fraction a major portion of the contaminating EPO and GM-CSF proteins as well as other contaminants. Following any one of subsequent alternative steps, i.e., isoelectric focusing (IEF) or MonoP chromatography or WCX HPLC chromatography of the second phase of the methodology, a virtually pure hMeg-CSF protein fraction is produced. While any one of the alternative isoelectric focusing, MonoP or WCX cation exchange HPLC steps may be selected following the preparative PAGE step as shown in Fig. 1A, the MonoP step is preferred since it is believed to generate an increase in quantity and activity of hMeg-CSF protein isolated downstream. The term "virtually pure hMeg-CSF protein fraction", is used herein to define a protein fraction which comprises at least about 90% protein and be virtually, if not totally, free of contaminating EPO and GM-CSF proteins. While contaminating EPO and GM-CSF proteins are believed to be totally removed from the virtually pure hMeg-CSF protein fraction, this virtually pure hMeg-CSF protein fraction is not purified to structural homogeneity at this point. A further "functionally ho oge- neous" sequenceable hMeg-CSF fraction is produced after reverse phase HPLC (See Fig. 1A) . The functionally homogeneous se¬ quenceable hMeg-CSF is eluted from this column with approxi¬ mately 50% acetonitrile and about 0.1% TFA. This functionally homogeneous hMeg-CBF is at a level of purity which has allowed for accurate N-terminal amino acid sequencing via standard techniques in the sequencing field as shown in Example 5 below. Moreover, this final functionally homogeneous hMeg-CSF has a specific activity of at least approximately 4 x 10 3 CFU-meg colonies/mg protein (this figures is the total activity of both the smaller and the larger species). In a preferred embodiment of the present invention, entirely homogeneous hMeg-CSF is produced (containing either one or both 24-35 and 50-70 kD species). After the preparative PAGE step described above, the hMeg-CSF is chromatofocused using a MonoP column (as described above) followed by reverse phase and cation exchange HPLC. It is believed that the isolated hMeg-CSF protein, which is purified to homogeneity, is produced in this embodiment following a reverse phase HPLC and a cation exchange HPLC step of the second phase of the method¬ ology as shown in Fig. IB. In other words, it is believed that the minor amount of contaminants present within the virtually pure hMeg-CSF protein fraction are removed by the chromato- focusing, reverse phase HPLC and WCX cation exchange HPLC steps, as illustrated in Fig. IB, to generate the isolated pure homogeneous hMeg-CSF protein. To isolate the hMeg-CSF protein(s) in accordance with either embodiment of the instant invention, urine is collected from patients with aplastic anemia and concentrated preferably by ultrafiltration. Alternatively, lyophilization and dialysis can be used to concentrate the urine. Use of Amicon YM10 membrane for ultrafiltration is particularly preferred (Amicon, Beverly, MA). Urinary salts are removed by molecular sieve chromatography (using, for Example Sephadex G-50 gel filtra- tion, Pharmacia, Piscataway, NJ). Other molecular sieve columns, such as BioGel P-10 (BioRad, Richmond, CA) or Bio Gel P-30 (BioRad) or Sephadex G-25 (Pharmacia, Uppsala, Sweden) can also be used. The pH and salt concentration of the peak protein pooled from the molecular sieve column is adjusted and the material is applied to an anion exchange support such as DEAE cellulose, (Whatman, Clifton, NJ) or DEAE Bio Gel A (Bio Rad, Richmond, CA) . The objective is to remove nonionic (non- binding) matter. Thus, e.g., non-binding proteins are removed by rinsing with e.g., phosphate buffer but binding proteins, including hMeg-CSF, are eluted with 0.15 M NaCl. Alternative¬ ly, the fraction(s) recovered from the separation according to molecular weight can be applied to a cation exchange support using e.g., CM-Sepharose (Pharmacia, Piscataway, NJ) or CM Bio Gel A (Bio Rad, Richmond, CA) . hMeg-CSF will also bind to these supports under the given conditions and is eluted in a similar fashion. The use of CM-cation exchange has yielded more hMeg protein and is preferred. In one embodiment, the active material recovered from either ion exchange support is preferably dialyzed extensively to remove interfering salts, lyophilized, resuspended and ready for further purification steps. These steps include prepara¬ tive polyacrylamide gel electrophoresis using native (non- detergent) conditions which generates a substantially pure hMeg-CSF protein fraction. This is followed by a step that separates the substantially pure hMeg-CSF protein fraction from residual impurities on the basis of its weakly basic pH to generate a virtually pure hMeg-CSF protein fraction. This step can be performed by either IEF in either liquid or immobilized phases or preferably MonoP. The MonoP chromatography results in recovery of virtually pure hMeg-CSF protein fractions when the pH of the eluting buffer is between 7.0 and 7.5. A virtu¬ ally pure hMeg-CSF protein fraction is detected in IEF gels at a pH ranging between about pH 7.2 and about pH 7.4. The virtually pure hMeg-CSF protein fraction obtained from either MonoP or IEF or WCX HPLC can be further purified by application to a C18 reverse-phase HPLC (e.g., from Beckman Instruments, Fullerton, CA), as shown generally in Fig. 1A. Alternatively, C8, C4 or Cl reverse phase HPLC columns can be used such as Brownlee RP-4, Brownlee Aquapore RP-300 or Brownlee RP-8 (Rainen, Woburn, MA), Vydec C-4 Protein/Peptide Column, Vydec C-8 (Rainen), Beckman Ultrasphere Octyl Column (Rainen) or a Cl + C4 Column (Pharmacia, Uppsala, Sweden). The hMeg CSF protein is eluted from the C18 column with approxi¬ mately 50% acetonitrile and about 0.1% TFA (trifluoroacetic acid) to produce functionally homogeneous hMeg-CSF, (i.e., the only hematopoietic activity detected is hMeg-CSF, but it is not constitutively homogeneous) . Functionally homogeneous hMeg-CSF protein is at a level of purity which allowed for accurate amino acid sequencing via standard techniques in the sequencing field illustrated in Example 5 below. In a preferred embodiment of the instant invention, homogeneous hMeg-CSF is produced. After recovery of the hMeg- CSF protein from the preparative PAGE step above, the protein is chromatofocused using a MonoP HR 5/20 column (Pharmacia) . The fractions containing hMeg-CSF activity are then further purified by chromatography on reverse phase HPLC using a C18 column followed by polyaspartate WCX cation exchange HPLC. Homogeneous hMeg-CSF protein is obtained after binding to a polyaspartate WCX cation exchange HPLC column and elution with substantially greater than 0.15M NaCl. Alternatively, any other reverse-phase and cation exchange HPLC columns as those skilled in the art will appreciate can be used. The buffers and conditions will vary but they can be ascertained by no more than routine experimentation by those of ordinary skill in the art. In an alternative preferred embodiment of the present invention, a streamlined purification procedure can be used to isolate hMeg-CSF. This procedure leads to the production of homogenous hMeg-CSF but requires much less manipulation of the protein. Aplastic anemia urine concentrate, obtained as described is dissolved in 100 ml of 0.8 M urea containing two micrograms/ml of the protease inhibitor leupeptin (Boehringer Mannheim) . The material is concentrated on a 10 6 molecular weight cut-off membrane such as Omegacell (Filtron Technology Corporation, Clinton, MA). The material retained on the membrane is discarded and the flow-through is collected and concentrated on a 10 5 cut-off membrane. The flow-through is collected and concentrated on a 10* cut-off membrane. hMeg-CSF contained in the 10* - 10 5 fraction is further purified by chromatography using a Polyaspartic acid WCX cation exchange column which may be repeated multiple times. hMeg-CSF elutes from the column at greater than 0.5 M NaCl. The material obtained using this streamlined isolation procedure appears to give a much better recovery of the biological activity of the hMeg-CSF protein and can form a single band on SDS-PAGE. The hMeg-CSF of the instant invention can be used in methods to treat mammals, such as to potentiate platelet forma¬ tion in patients with thrombocytopenia or atherosclerosis, in wound healing, in patients with antibody to platelets, or to make drugs to enhance, alter or possibly decrease platelet function. For example, structural analogs, which are true analogs of hMeg-CSF, can be produced based on a detailed characterization of the binding of hMeg-CSF to its cellular receptor. These analogs would bind tightly to the receptor without activating it and, thus, block the biologic activities of hMeg-CSF. hMeg-CSF may be used in the treatment of diseases characterized by a decrease in the level of hematopoietic cells, particularly those of the megakaryocytic lineage. It may be used directly to stimulate megakaryocyte and platelet production and may indirectly stimulate other hematopoietic lineages. Among conditions susceptible to treatment with hMeg- CSF protein of the present invention is thrombocytopenia, a reduction in the number of circulating platelets in peripheral blood. Thrombocytopenia may be induced by exposure to certain viruses, drugs or radiation. It is often a side effect of various forms of cancer and/or AIDS therapy, e.g., exposure to chemotherapeutic drugs. Therapeutic treatment of throm¬ bocytopenia with hMeg-CSF protein compositions may avoid un- desirable side effects caused by treatment with presently available drugs. The amount of hMeg-CSF to be used will, of course, depend upon the severity of the condition being treat¬ ed, the route of administration chosen, the specific activity of the hMeg-CSF protein and the responsiveness of the individu- al patients' receptors to the hMeg-CSF protein, and ultimately will be decided by the attending physician or veterinarian. Such amount of hMeg-CSF protein as determined by the attending physician or veterinarian is also referred to herein as a "treatment effective" or a "therapeutically effective amount". Typical hMeg-CSF protein treatment effective amounts are contemplated to be in the range of about 0.1-100, preferably about 1-50, and more preferably about 2-15 units of hMeg-CSF protein/kg body weight, for a period of about 3-60 days, and preferably 15-45 days. While the isolated -pure homogeneous hMeg-CSF protein is preferred, use of the virtually pure or functionally homogeneous hMeg-CSF protein fractions for treat¬ ing mammals including humans and forming medicaments are also contemplated by the instant invention as these hMeg-CSF protein fractions can be rendered pyrogen-free using, for example polymyxin B resins to remove endotoxins (if present) as is well known in the art. Either or both higher and lower m.w. species can be used. Alone, or in combination with other hematopoietins, hMeg-CSF may enhance hematopoiesis. The present invention may also be employed, alone or in combination with other hematopoietins, in the treatment of other blood cell deficien¬ cies or anemia (red cell deficiency). Such therapeutic compo¬ sitions may also be administered in conjunction with other human factors. A non-exclusive list of other appropriate hematopoietins including CSFs and interleukins for interaction with the hMeg-CSF protein of the present invention includes GM- CSF, G-CSF, M-CSF, erythropoietin (EPO), IL-1, IL-2, IL-3, IL- 4, IL-5, IL-6, IL-7 (all available from Amgen, Thousand Oaks, CA) , and IL-9 (obtainable as described in Yang, Y-C et al., Blood 74, 1880-1884, 1989). Other appropriate factors include without limitation Activins and Inhibins (Genentech, South San Francisco, CA), TPO/TSF (obtainable as described in McDonald, T.P. et al., Int. J. Cell Cloning 7:139-155, 1989), IL-11 (obtainable as described in Paul, S.R. et al. Proc. Natl. Acad. Sci. USA 87:7512-7516, 1990), LIF (Amgen) and SCF (obtainable as described in Martin et al. Cell £3:203-211, 1990). Other uses for the hMeg-CSF protein are in the treatment of patients recovering from chemotherapy, therapeutic radiation, bone marrow transplants, enhancing host blood clotting abilities during surgery and in burn patients. hMeg-CSF may also be employed to develop monoclonal and polyclonal antibodies generated by standard methods for diagnostic, hMeg-CSF gene therapeutic use, or research reagent use (e.g., in the iden¬ tification of the hMeg-CSF gene and in screening recombinant microorganisms producing hMeg-CSF). Therefore, yet another aspect of the invention is directed to pharmaceutical formulations or dosage forms for treating the conditions referred to above. Such pharmaceutical formulations comprise a therapeutically effective amount of the hMeg-CSF protein(s) of the present invention in admixture with a pharmaceutically acceptable carrier, such as sterile water, sterile normal saline, dextran, parabens, citrates, stearate calcium or the like. Such compositions can be systemically administered either parenterally, intravenously or subcutane- ously, if appropriate, although oral or inhalable delivery systems suitable for protection of proteins from the gastric environment are also contemplated. Examples are U.S.P. No. 4,925,673; 4,624,251; and 3,703,173. When systemically admin¬ istered, the pharmaceutical formulations for use in the present invention are preferably in the form of pyrogen-free, paren- terally acceptable aqueous solutions. The preparation of such a parenterally acceptable protein solution, having due regard to pH, isotonicity, stability and the like, is well within the skill of the art. While it is possible for the novel hMeg-CSF to be administered as the pure homogeneous protein, functional¬ ly homogeneous or virtually pure protein fraction, it is generally preferable to present it as a pharmaceutical formula¬ tion or preparation. The formulations of the present invention, both for veterinary and for human use, therefore comprise a hMeg-CSF protein, as described above, together with one or more phar¬ maceutically acceptable carriers or diluents or excipients thereof and optionally other therapeutic ingredients. The carrier(s) must be physiologically "acceptable" in the sense of being compatible with the other ingredients of the formulation, if any, and not deleterious to the recipient thereof. Desir¬ ably, the formulation should not include oxidizing agents and other substances with which peptides are known to be incom¬ patible. The formulations may conveniently be presented in unit dosage form and may be prepared by any of the methods well known in the art of pharmacy. All methods include the step of bringing into association the active ingredient with the carrier which constitutes one or more accessory ingredients. In general, the formulations are prepared by uniformly and intimately bringing into association the active ingredient, e.g., a hMeg-CSF protein, with liquid carriers or finely divided solid carriers or both, and then, if necessary, shaping the product into the desired formulation. Non-limiting exam¬ ples of carriers which can be used in the pharmaceutical formulations of the present invention include human serum albumin, dextran-imbedded hMeg-CSF, or other biocompatible polymers degraded in the body. Time- and sustained-release implants could also be used as delivery systems. The new gene therapy technology could also be used to insert the gene(s) for hMeg-CSF into autologous blood cells which are then reinjected in the patient as in vivo production sites. Rosenberg, R.D. et al. ____• Formulations suitable for parenteral administration conveniently comprise sterile aqueous solutions of the active ingredient, e.g., isolated pure homogeneous hMeg-CSF protein, functionally homogeneous or virtually pure hMeg-CSF protein fraction, with solutions which are preferably isotonic with the blood of the recipient. Such formulations may be conveniently prepared by dissolving solid hMeg-CSF proteins in water to produce an aqueous solution, and rendering said solution sterile. The resultant solution may be presented in unit or multi-dose containers, for example sealed ampoules or vials. It will be appreciated that the pharmaceutical formulations or dosage forms of the present invention need not contain an effective amount of the hMeg-CSF of the present invention as such effective amounts can be achieved by adminis¬ tration of a plurality of such formulations or dosage forms. The present invention also contemplates the use of biologically active fragments of hMeg-CSF. Biologically active fragments (substantially smaller than the entire hMeg-CSF protein molecule) of hMeg-CSF can be obtained for example by limited proteolytic digestion of the hMeg-CSF protein (either purified to homogeneity using techniques described above or purified recombinant protein as described in Example 7 below) using e.g., trypsin, papain, chymotrypsin, V-8 protease, endo- proteases or other proteases well-known in the art or other agents such as cyanogen bromide (which cleaves after methionine residues) and testing for hMeg-CSF activity after isolation in, e.g. , the in vitro murine fibrin clot assay of Example 3 below. It is anticipated that active fragments of the hMeg-CSF pro¬ tein, substantially smaller than the entire hMeg-CSF protein, can be identified. Alternatively, once the full-length cDNA clone encoding hMeg-CSF has been obtained as described in Example 6 below, limited nuclease digestion of the cDNA using e.g., Bal-31 nuclease, can be performed and the resultant product cloned and expressed as taught in Examples 6-8 below. The protein product can then be tested for activity, leading to the identification of active hMeg-CSF fragments. In the examples presented below, the free-amino terminal amino acid sequence of hMeg-CSF (obtained using the "functionally homogeneous" protein fraction) is presented. Also disclosed are methods for completing the sequencing of hMeg-CSF and for identifying the gene encoding the hMeg-CSF of the present invention, cloning it and expressing recombinant hMeg-CSF. In addition data are presented showing the purity of hMeg-CSF produced in accordance with the methods of the present invention. It will be apparent to those skilled in the art that many different organisms (including other bacterial species, yeast or mammalian cells) and vectors other than those described in the specific examples below can be used to identi¬ fy, clone and express recombinant hMeg-CSF. Non-limiting examples of such organisms and vectors can be found in, e.g., Sambrook, J. et al.. Molecular Cloning : A Laboratory Manual 2nd edition. Cold Spring Harbor Press, NY, 1989. The present invention is further described below in specific examples which are intended to illustrate the inven¬ tion without limiting its scope. EXAMPLE 1: PURIFICATION OF hMEG-CSF PROTEIN FROM URINE OF APLASTIC ANEMIA PATIENTS Daily urine from patients with aplastic anemia was collected and mixed with about 50% phenol in ethanol to a final concentration of about 0.1% phenol. The aplastic anemia patients had pancytopenia with hypocellular bone marrow and no signs of systemic or urinary tract infections. Platelet counts were less than about 20,000/mm 3 , leukocyte counts were less than about 1,500/mm 3 and hemoglobin concentration was maintained around 6g/dl by blood transfusions. The purification scheme in Fig. 1A recites generally the purification steps described below. A. Ultrafiltration The collected urine was filtered and concentrated on a YM10 filter (10,000 molecular weight cut-off, obtained from Amicon, Beverly, MA) by ultrafiltration. B. Gel Filtration Chromatography The ultrafiltered urine was desalted using a 10 x 100 cm column of Sephadex G-50 (obtained from Pharmacia, Piscata- way, NJ). The initial protein peak (the exclusion effluent) was collected. C. Ion-Exchange Chromatography The G-50 pool was adjusted with about 0.02 M NaH 2 PO < , and NaCl to pH 4.8-5.0. DEAE-Cellulose (DE-22, obtained from Whatman, Clifton, NJ) equilibrated with about 0.025 M NaH 2 PO « was added to the G-50 pool (about 1 g dry powder/1 liter urine) and stirred for about 30 min. The mixture was allowed to settle for about 2 hours at about 4"C and poured into a column with gentle suction. The absorbed protein fraction was eluted with about 0.05 M NaH 2 P0 4 with about 0.15 M NaCl. The eluted sample was lyophilized to dryness and stored at about -70°C. Alternatively, the concentrated, desalted urine from the G-50 column was loaded on a CM-Sepharose Fast Flow column (obtained from Pharmacia) and equilibrated with about 0.01 M phosphate buffer, pH of about 5.5. The bound ionic protein material (including hMeg-CSF protein) was eluted with about 0.05 M Na 2 HP0 4 and about 0.15 M NaCl. The eluted material was then dialyzed and lyophilized. D. Dialysis The Crude Urinary Extract obtained above was dis¬ solved in about 30 ml/g and dialyzed for about 2 days against about 4 liters of water (the water was exchanged three times a day), using dialysis tubing, such as Spectra/Por, (Spectrum Medical Industries, Inc., Los Angeles, CA) with a molecular weight cut-off of approximately 10 kDa. After dialysis, the material was lyophilized and stored at about -70 β C. The protein content was determined using the BioRad method on a sample of the urinary extract (Bradford, M. Anal. Biochem. 12:248, 1976). E. Preparative Polyacrylamide Gel Electrophoresis The native preparative gel was about 5% polyacryla- mide (about 30% T and about 2.6% C) in about 0.05 M Tris-Hcl (pH 6.8). The 14 x 16 x 0.3 cm gel had no stacking gel, and about 0.5 g dry powder of the freshly dialyzed, lyophilized extract was added per gel. The upper and lower reservoir buffer contained about 0.025 M Tris-glycine (pH 8.3). The gel was run at about 50 mA per gel until the front was about 13 cm from the top. The gel was removed from the apparatus and cut into about 1 cm slices which were homogenized, the protein was eluted by adding about 20 ml water to the homogenized gel and incubating at about 4 β C overnight. The supernatant was removed and about 20 ml water again added overnight. Both supernatants were pooled, dialyzed for two days and lyophilized. Slice numbers #2-5, which contain substantially pure hMeg-CSF protein fractions, were pooled and used for the next step. Fig. 2 illustrates the substantially pure hMeg-CSF protein fractions and biological activity of 1 cm slices of the 5% PAGE, with fraction #1 being at the top of the gel. F. Isoelectric Focusing Isoelectric focusing (IEF), using a pH range of about 3.5-10, documents that the pl of hMeg-CSF protein in glycosy¬ lated and sialyated form was about 7.2-7.4 (Fig. 3). The 115 x 0.2 x 25 cm gel is composed of about 5.4% acrylamide, about 0.144% Bis-acrylamide and ampholines (pH equal to about 3.5-10) with the sample as part of the gel composition. G. MonoP Chromatography A MonoP HR 5/20 column (Pharmacia) was equilibrated with about 0.025 M Triethanolamine (brought to a pH of about 8.3 with acetic acid) at a flow rate of about 1 ml/min. The sample, about 70 mg of substantially pure hMeg-CSF protein from the preparative PAGE step, is preferably filtered (0.22 mi- crons) and injected onto the column. The elution buffer of about 9% Polybuffer 96 (Pharmacia) and about 0.21% Phar alyte (Pharmacia), pH 8-10.5, (brought to pH of about 6.0 with acetic acid) was started at a flow rate of about 1 ml/min. This gives a pH gradient of about 8 to 6. Fractions were collected every min for 40 mins. The fractions eluting at 16-20 min contain virtually pure hMeg-CSF protein and had a pH of about 7.0-7.5 (Fig. 4). These fractions were pooled, dialyzed for two days and lyophilized. H. Polyaspartic Acid WCX Cation Exchange HPLC The hMeg-CSF protein fractions (about 0.5 mg) from the MonoP column were resuspended in about 0.05 M phosphate buffer (pH of about 6.45) and filtered (0.22 microns). The sample was injected into a Polyaspartic Acid WCX HPLC column (The Nest Group, Southboro, MA; 4.6 mm x 10 cm) and about a 30 minute gradient of about 0-0.5 M NaCl followed by a 5 minute gradient of 0.5-1 M NaCl in about 0.05 M phosphate buffer (pH of about 6.45) was used to elute the hMeg-CSF protein fractions which were present in fractions #13-17 (i.e., at about 0.5-1 M NaCl). I. C18 Reverse-Phase HPLC The virtually pure hMeg-CSF protein fraction(s) obtained from the MonoP were resuspended in about 0.1% TFA (trifluoroacetic acid) in water. The sample was filtered (0.22 microns) and injected into a C18 reverse-phase HPLC column (Beckman Instruments, Inc., San Ramon, CA; Ultrasphere ODS; 4.6 mm x 24 cm). The column was equilibrated with about 0.1% TFA and about 30% acetonitrile for about 10 min and the protein eluted with a 30 min gradient from about 30-70% acetonitrile with 0.1% TFA at about 1 ml/min. One minute fractions were collected. The fractions eluting at 32-34 min (#32-34) contain functionally homogeneous hMeg-CSF protein, (see Fig. 5), and were pooled and lyophilized. In a preferred alternative embodiment of the present invention, the hMeg-CSF is purified using steps A-E above. The preparative PAGE step (Step E) is followed by chromatofocusing using a MonoP HR 5/20 column as in step G above. The active fractions from the preparative PAGE step were pooled, dialyzed for at least two days against distilled water, lyophilized, resuspended in 0.025M Triethanolamine , pH 8.3, and filtered (0.22 microns) before injection into the MonoP column. This is followed by C18 reverse phase and WCX polyaspartate cation exchange HPLC, performed as described in steps I and H above, respectively and further below. F'. Chromatofocusing Chromatofocusing, using a Mono P HR 5/20 column was performed as described above in G. The active fractions were pooled, lyophilized, dialyzed and resuspended. G' . C18 Reverse Phase HPLC The hMeg-CSF fractions recovered from the MonoP HR 5/20 column were treated as described above. The data are presented in Fig. 5. H' . Polyaspartic Acid WCX Cation Exchange HPLC The hMeg-CSF protein fractions from the C18 Reverse Phase HPLC column were lyophilized and resuspended in about 0.05 M phosphate buffer (pH of about 6.45) at a concentration of about lmg/ml protein. The sample was injected into a Polyaspartic Acid WCX HPLC column (The Nest Group) and ran as described above in H. The material eluted from the WCX column was dialyzed, lyophilized and loaded in a single lane of a 12% non-reduced SDS-PAGE gel, electrophoresed, transferred to an Immobilon PVDF membrane (Millipore Corp., Bedford, MA) and stained with Coomassie blue. The resulting single band at 2.8 cm from the top of the gel ran at a molecular weight of approximately 50-70 kd as shown in Figure 8. This may represent a dimer due to the non-reducing conditions under which the gel was run, but may also represent a fragment or another species having Meg-CSF activity. EXAMPLE 1A: STREAMLINED ISOLATION PROCEDURE Homogeneous hMeg-CSF may be obtained using the following procedure. Ultrafiltration: 50 liters of aplastic anemia urine concentrate is dissolved in 100 ml of 0.8 M Urea containing two micrograms/ml of the protease inhibitor leupeptin (obtained from Boehringer Mannheim) . The material is concentrated on a 10 6 molecular weight cut-off membrane (Omegacell, Filtron Technology Corpora- tion, Clinton, MA). Material retained on the 10 6 membrane was discarded. The flow-through was collected and concentrated on a 10 5 cut-off membrane. The flow-through was collected and concentrated on a 10* cut-off membrane. The concentrate in each step was washed with three volumes of 0.8 M Urea plus two micrograms/ml leupeptin. Human megakaryocyte colony-stimu¬ lating activity was detected in the 10 5 - 10 6 fraction and in the 10* - 10 5 fraction. The 10 5 - 10 6 fraction has also been found to contain erythropoietin and GM-CSF activities. The material contained in the 10* - 10 5 fraction was further puri- fied by weak cation exchange HPLC using a polyaspartic acid WCX column (The Nest Group, Southboro, MA) . WCX HPLC Chromatography: The 10* - 10 5 fraction was adjusted to a pH of 6.0 before injection into the WCX HPLC column. The column (100 X 4.6 mm; 5 microns) was equilibrated with 0.05 M sodium phos¬ phate buffer, pH 6.0. A flow rate of 1.0 ml/min was used. A two phase gradient of 0 to 0.5 M NaCl in 0.05 M sodium phos¬ phate, pH 6.0, over 20 minutes removed contaminating proteins, immediately followed by a gradient of 0.5 to 1 M NaCl in sodium phosphate buffer over 5 minutes to elute hMeg-CSF. 2.5 ml fractions were collected. hMeg-CSF activity was detectable in pool A (fraction nos. 1-6), pool D (fractions nos. 16-21) and ' pool E (fractions nos. 22-28) as shown graphically in Fig. 9. Alternatively, individual fractions from two HPLC runs were concentrated by lyophilization and hMeg-CSF activity was recovered from fraction nos. 16-21 as shown in Fig. 10B. hMeg- CSF eluted with greater than 0.5 M NaCl with peak activity in fractions no. 19, 20 and 21. A pool of material from fraction nos. 16-32 from the WCX HPLC column was electrophoresed on a 12% analytical SDS- PAGE under both reducing (5% 2-mercaptoethanol) and non-reduc¬ ing conditions. The gel was sliced in 1 and 0.5 cm slices and the protein eluted by two consecutive overnight incubations at 4°C in IMDM medium plus 1% FCS (1 ml/cm gel each). Pre-stained molecular weight standards were run in adjacent lanes to determine the molecular weight. In this particular experiment, a single band spanning the size range of about 57-66 kD was detected within the expected 50-70 kD range of hMeg-CSF activi¬ ty, as shown in Fig. 10A. (Other experiments have resulted in 52-55 kD single bands.) The eluted medium was pooled, dialyzed against distilled water for two days and assayed for biological activity. As shown in Fig. 11A, hMeg-CSF was detected under non-reducing conditions both at molecular weights of about 24- 35 kDa and about 50-70 kDa but most of the activity was associ- ated with the 50-70 kD species. Under reducing conditions (2- mercaptoethanol added) of the same material, hMeg-CSF was also detected at 24-35 kDa and 50-70 kDa as shown in Fig. 11B. Under reducing conditions however, the bulk of the activity is shifted from the higher molecular weight species to the lower molecular weight species. EXAMPLE 2: Purity of Meg CSF Human urine contains, in addition to Meg-CSF, other cytokines and growth factors including EPO, M-CSF and G-CSF (Das, S.K. et al.. Blood 58: 630-641, 1981; Miyake, R. et al., J. Biol. Chem. 252: 55582-5564, 1977; Kohsaki, M. et al., Proc. Natl. Acad. Sci. USA 81:3802-3806, 1983). A systematic explo¬ ration of the biological activities present in Meg-CSF prepara¬ tions from each of the purification steps was performed. The results presented here show that Meg-CSF purified through the MonoP step is virtually pure - the only biological activity detectable by assay being Meg-CSF. The specific activities are set forth in Table I below. TABLE I Specific Activities Purification CFU-Meg muCFU-gm + IL-6 EPO Steps (colonies/mg) (U/mg) (ng/mg) (U/mg) n.t. not tested * background levels of murine CFU-gm colonies # lower limit of EPO deductibility + murine granulocyte/macrophage colony forming units The assays performed were: * M-07-e according to Avanzi, G.C. et al. J^ Cell Physiol.. 1990, 145_:458-464. This Assay detects as little as 12.5 u/ml of GM-CSF, 6 u/ml of IL-3 and 5 u/ml of IL-9. * CFU-gm assays (which detect down to about 10 u/ml of G-CSF and M-CSF) according to Du D-L et al. Invest. New Drugs. 1991, £:149-157. * Quantikine 1 " (R&D Systems, Minneapolis, MN) which can detect as little as 6.5 pg/ml of IL-6 and 31.3 pg/ml of IL-1 alpha. * EPO assay according to Krystal, G. Exp. Hematol. , 1983, 11:649-660 which detects as little as 0.05 u/ml of EPO. The aplastic anemia (AA) urinary extracts and the fraction separated by chromatographic procedures were tested for EPO activity using a murine splenocyte 3 H-thymidine assay for EPO (Krystal, G., Exp. Hematol. 11:649-660. 1983). EPO presents the largest and most vexing contaminant. The same steps that take advantage of the weekly basic nature of Meg-CSF also separate Meg-CSF and EPO (which has a Pi of approximately 3.5). It has been determined that as little as 0.7 U/mg EPO is capable of stimulating megakaryocytic colonies in fibrin clot assays. Although 0.3 U/mg EPO does not stimulate megakaryocytic colonies on its own, it will enhance the number of colonies when added together with Meg-CSF purified by IEF. Therefore, even the preparative PAGE step in Table I, contains enough EPO to result in a higher specific activity for the EPO- contaminated hMeg-CSF preparation being tested to contribute to CFU-Meg stimulation, but the megakaryocyte stimulating activity of the MonoP and C18 steps is ascribable only to Meg-CSF. EXAMPLE 3: In vitro hMeg-CSF ASSAY The murine fibrin clot culture system was used to detect megakaryocyte progenitors (CFU-Meg) as described in Kuriya, S-I, et al., Exp. Hematol. 15:896-901, 1987. Briefly, Iscove's modification of Dulbecco's medium (IMDM, Sigma Chemical Co., St. Louis, MO) and samples were mixed with about 20% fetal calf serum (FCS, HyClone Laborato¬ ries, Logan, UT), marrow cells (final concentration about 5 x 10 5 cells/ml), about 20% fibrinogen (Sigma Chemical Co.) and about 10% thrombin (Sigma Chemical Co.). Aliquots of approxi¬ mately 0.4 ml were put in the center of 35 mm dishes and, when the clots were solid, about 0.6 ml IMDM was placed around them. After about 6 days, the fibrin clots were dried and stained for acetylcholmesterase (AChE) activity, as disclosed in Karnov- sky, M. , et al., J. Histochem. Cvtochem. 12:219 (1964), and Jackson, C.W., Blood 4_2:413 (1973). Colonies containing 3 or ' more AChE-positive cells were counted. The mean and the standard error were determined for triplicate plates. 5 The protein content of the samples was determined and the lyophilized samples were resuspended in Iscove's medium to a concentration of about 1 mg/ml and sterilized by filtration (0.22 microns). The samples were tested at three concentra¬ tions: about 6.7, 3.3 and 0.67 micrograms/ml assay mixture. 0 The results are set forth in Table II hereinbelow. Table II - Results of Murine Bioassay for Meg-CSF Specific Activity Purification Steps (units/mg Protein) 1 1) Sample obtained from the DEAE- 1.7 x 10* Cellulose pathway in accordance with the instant invention 2) Sample obtained from the DEAE- 4.3 x 10* Cellulose and preparative PAGE pathways in accordance with the instant invention 3) Sample obtained from the DEAE- 7.1 x 10 2 Cellulose, preparative PAGE and MonoP pathways in accordance with the instant invention 1 One unit is defined as one megakaryocyte colony in the murine fibrin clot assay. The Specific Activity of the DEAE- Cellulose sample (1) and the preparative PAGE sample (2) is reported in Table II as having greater activity than the MonoP and C18 HPLC samples (3 and 4, respectively). This is believed to be due to the contaminating proteins, such as EPO, GM-CSF, etc, in the DEAE-Cellulose and preparative PAGE samples (1 and 2, respectively). 4 ) Sample obtained from the DEAE- 4.0 x 10* Cellulose, preparative PAGE, MonoP and C18 reverse phase HPLC pathways in accordance with the instant invention (Fig. 1A) 5 ) Sample obtained from the DEAE- Cellulose, preparative PAGE, MonoP (Protein concen¬ Chromatofocusing, C18 reverse phase tration too low and WCX HPLC pathways in accordance to measure. ) with the instant invention (Fig. IB) EXAMPLE 4: In vivo hMeg-CSF ASSAY In vivo bioassays for hMeg-CSF will be performed according to the protocols described in Kuriya, S., et al., (EXP. Coll. Biol. 55:257-264, 1987) and Kuriya, S., et al., (Experimental Hematology Today. Bau S.J., et al. eds., pp. 33- 38, Springer-Verlag, New York 1985). In the above-cited references, native hMeg-CSF (present in "crude urinary extract" form) significantly increased the number of circulating plate¬ lets in rats when compared to saline-injected controls. There was no change in platelet size. Groups of 5 rats will be injected intraperitoneally with doses of hMeg-CSF that produce about 20-30 colonies per 2xl0 5 cells in the mouse CFU-Meg assay. Injections will be administered for three consecutive days. The animals will then be bled from the tail vein after twenty-four hours and the number and size of platelets determined by Coulter Counter. Controls will consist of animals injected with: 1) super- natants or lysates of cells not transfected in hMeg-CSF cDNA; 2) physiological saline; and/or 3) non-human hMeg-CSF-contain- ing fractions devoid of other cytokines known to stimulate CFU- Meg. The in vivo activity can also be determined by measuring the number of megakaryocyte progenitor cells of mice after treatment with hMeg-CSF. Five mice will be injected with a single dose of purified natural or recombinant hMeg-CSF. The animals will be sacrificed after 2, 4 and 6 days post-inocula¬ tion and their spleens and femurs will be removed aseptically. Single cell suspensions will be made by disrupting the spleen and flushing the marrow from the femur. The cells will then be plated in the fibrin clot assay in place of the murine marrow cells and stimulated with pokeweed mitogen spleen conditioned medium (PWM-SCM), a source of Meg-CSF. It was previously shown (Kuriya, S. et al., supra) that "crude urinary extract" Meg-CSF increased the number of murine splenic megakaryocyte progeni¬ tors as evidenced by an increase in colonies produced by the spleen cells compared to saline controls. There was no in¬ crease in bone marrow megakaryocyte progenitors, perhaps due to the low concentrations of Meg-CSF employed. Therefore, the purified hMeg-CSF of the present invention (both naturally derived and recombinant) is anticipated to increase the number of spleen megakaryocyte progenitors and increase the number of bone marrow progenitor cells in animals receiving the hMeg-CSF of the present invention. EXAMPLE 5: Sequencing of the hMeg-CSF Protein The "functionally homogeneous" hMeg-CSF protein fraction described above was used to determine the putative N- terminal amino acid sequence which may be included in Meg-CSF thus demonstrating that preparations within the present inven¬ tion are useful to sequence hMeg-CSF. The protein sequence was determined from protein immobilized on PVDF membranes described above after electrotransfer from a SDS-PAGE (electrophoresed under non- reducing conditions) as described in Matsudaira, P., J. Biol. Chem. 262:10035-10038. 1987 and Hunkapiller, M. et al.. Methods in Enzymol. £1:227-236, 1983, using Gas-Phase Edman sequencing as described in Hewick, R.M. et al. J. Biol. Chem. 256:7990- 1991 , 1981. The equipment used was Porton Model 2020 gas phase protein sequencer (Tarzana, CA) . The N-terminal amino acid sequence was X-Asp-Pro-Val- Glu-Ser-Pro-Val-Pro-Y, wherein X and Y are undetermined resi¬ dues. The species thus partially sequenced is the higher molecular weight (50-70 kD) species. EXAMPLE 6: Molecular Cloning and Derived Nucleotide Sequence of Human Meg-CSF cDNA Due to the extremely low level of Meg-CSF expression, it is impractical to obtain large amounts of the cytokine from natural sources for biochemical studies or clinical trials. However, purification of hMeg-CSF to homogeneity and amino acid sequence information for Meg-CSF, as shown in Example 5 above, have opened the way for molecular cloning of DNA sequences encoding the amino acid sequence for Meg-CSF. Cloning can be performed for example as follows: (i) Degenerate deoxyoligonucleotide sequences (DOS) corresponding to all codon combinations of the N-terminal amino acid sequence will be employed as primers to amplify Meg-CSF specific mRNA by polymerase chain reaction (PCR) and as probes to screen and finally clone Meg-CSF cDNA. (ii) Peptides corresponding to N-Terminal Meg-CSF will be synthesized and conjugated to carrier molecules. The polypeptide conjugates will be used to make anti-peptide antibodies which recognize hMeg-CSF using techniques well known by those skilled in the art which would also provide an alter¬ nate approach to obtaining Meg-CSF cDNA via immunological screening of an expression cDNA library. PCR Amplification of Meg-CSF Specific cDNA Using DOS Probes and Molecular Cloning Selective amplification and subsequent cloning of specific full-length (or near full length) cDNA can be ac¬ complished by the following steps: (1) First strand synthesis can be initiated by use of oligo-dT primer (which binds to the polyA tail found on eukaryotic mRNAs) , second strand cDNA synthesis can be accomplished by the Klenow fragment of DNA polymerase 1. (2) PCR utilizing a specific DOS primer and an oligo-dT primer can be used to amplify the cDNA of interest prior to cloning into Ml3 phage or other vectors. (3) A specific cDNA library can be screened with 32 P-labeled DOS primers and positive clones will be characterized by restric¬ tion mapping and sequencing. (A variation of this approach would use random primers to initiate the synthesis of first cDNA) . The second strand can then be synthesized in the same manner as above and the double stranded cDNAs can be cloned into an appropriate vector (e.g., the lambda ZAPII vector (Stratagene, La Jolla, CA) . Subsequently, DNA can be isolated from the entire lambda ZAPII cDNA library and Meg-CSF specific clones will be amplified by PCR utilizing a specific DOS and a vector-based primer. The amplified products will then be cloned into e.g., an M13 vector (GIBCO/BRL Life Technologies, Inc., Gaithersburg, MD) for characterization. This technique is described in Sambrook, J. et al., supra. PCR Amplification of Meg-CSF Specific mRNA and Cloning The primer pair to be used for Meg-CSF mRNA signal amplification will be a first strand oligo-dT "universal" primer which binds to the sense strand, and a second strand specific primer (DOS A) which binds to the antisense strand. The DOS A is a 17-mer which corresponds to all codon combina¬ tions derived from the N-terminal amino acid sequence of Meg- CSF, and has the sequence DOSA 1: GAC/T CCN GTN GAA/G TCN CC; DOSA 2: GCT/C CCN GTN GAA G TGC/T CC wherein alternative third bases are represented by an oblique (C/T means C or T) and N denotes any one of the four bases. In addition, another specific primer (DOS B), whose sequence is based on a stretch of amino acids adjacent to and downstream from (just carboxyl to) the DOS A primer, will be synthesized and used as a confirmatory hybridization probe. The DOS B sequence will be TCI CCN GTN CCN GAG/A wherein I=inosine base. Knowledge of the origin of Meg-CSF expression is of crucial importance in successfully cloning this factor. Therefore, T cells and spleen cells (Ogata, K. et al.. Blood 74: (Suppl. 1) :330a. 1989) together with endothelial tissue, placenta, stromal cells and other tissues (e.g., liver) that potentially produce Meg-CSF will be examined for their Meg-CSF mRNA expression according to Cantrell et al., PNAS (USA) 12:6250-54, 1985; Bennett et al., PNAS (USA) 11:7512-16, 1990; Martin et al.. Cell £3:203-211, 1990. Due to likely low levels of Meg-CSF gene expression, the Meg-CSF specific signal will be amplified by PCR before detection. Total RNA will be extracted from different tissues (Chirgwin, J.M. et al., Biochem. 11:5294-99, 1979) and poly A + RNA will then be isolated by two cycles of chromatography on an oligo-dT cellulose column (Sambrook, J. et al.. Molecular Cloning : A Laboratory Manual. Second edition. Cold Spring Harbor Press, NY, 1989). The first strand cDNA will be synthe- sized using Mouse Moloney Leukemia Virus (M-MLV) reverse transcriptase (GIBCO/BRL Life Technologies) with an oligo-dT primer, and the second strand using the Gubler and Hoffman procedure (Gubler, U. et al., Gene 25:263-269, 1983; Dai, W. et al., Biochem. , Biophys. Res. Comm. 168: 1-8, 1990). The double stranded cDNAs will then be subjected to 30 cycles of PCR (Perkin-Elmer Cetus) amplification, as adopted from described procedures (Saiki, R.K. et al.. Science 239:487-491. 1988; Gyllensten, U.B. et al., Proc. Natl. Acad. Sci. USA 85:7652- 7656, 1988; Loh, E.Y. et al.. Science 243:217-20. 1989; Cooper, D.L. et al., Biotechniques 9:60-65. 1990; Dorfman, M. et al., Biotechniques 1:568-570, 1989) using DOS A and oligo-dT prim¬ ers. PCR-amplified cDNAs from different tissue sources will be analyzed by either dot-blot procedures or agarose gel electrophoresis followed by blotting onto reinforced nitrocel- lulose membranes such as those available from Schleiker and Schul, Keene, NH. After baking under vacuum at 80°C for 2 hrs, the nitrocellulose blot will be probed sequentially with 32 P- labeled DOS A and B. The tissue that gives the strongest specific signal to probe DOS A and B will be used as the primary source for Meg-CSF cDNA cloning. Double stranded cDNA synthesized and amplified as described above will be methylated with Eco RI methylase, blunt-ended by incubation with Klenow enzyme and dNTP, and ligated with Eco RI linkers (GIBCO/BRL Life Technologies). After digestion with Eco RI, cDNAs will be separated by size on an agarose gel. The cDNAs larger than the minimal coding sequence will be eluted by electroelution and ligated with the M13 mpl9 phage vector which has been cut with Eco RI and dephosphorylated. The ligated products will be transformed into CaCl 2 treated E_i- coli JM 101 cells and plated on YT/IPTG/X-gal plates. Screening for Meg-CSF cDNA clones will be accomplished by hybridization with both DOS probes (DOS A and B). M13 phage plaques grown overnight will be lifted onto nitrocellulose membranes. The membranes will be processed as described (Caplan, H.S. et al., J. Biol. Chem. 263:332-339. 1988; Dai, W. et al., Jj. Biol. Chem. 1990, 2J>j>: 19871-19877 ) and hybridized with 32 P-ATP labeled DOS probes. Plaques that hybridize to both probes will be selected. Single stranded M13 mpl9 DNA will be prepared as a template from positive clones and sequenced by the dideoxynucleotide chain termination method (Sanger, F. et al. , Proc. Natl. Acad. Sci. USA 14:5463-5467, 1977). This sequence information will confirm the identity of the putative Meg-CSF clones. PCR Amplification of the Meg-CSF Specific Sequence from a ZAP II cDNA Library As a variation to the above approach, random primers will be used for the first strand cDNA synthesis. This ap¬ proach is of particular importance if the size of Meg-CSF mRNA is more than 3 kb. Following methylation, linker addition, and Eco RI digestion, double stranded cDNA will be ligated into the lambda ZAP II vector (Stratagene) to make a complete cDNA library. The entire cDNA library will be amplified once for DNA isolation and the amplified phage library will be then eluted. Recombinant phage DNA will be prepared (Sambrook, J. et al., supra) and used as the starting material for PCR amplification. Meg-CSF specific cDNA cloned in the lambda ZAP II vector will be amplified by 30 cycles of PCR using a specif¬ ic DOS A primer and a second primer corresponding to the lambda ZAP II phage sequences (containing a Hind III site at the multicloning site) located in the lac Z gene. Following amplification, DNA will be blunt-ended with Klenow enzyme in the presence of dNTP. Blunt-ended DNA will then be cut with Hind III and ligated to a Hind III/Sma I cut M13 phage vector (GIBCO/BRL Life Technologies). The ligation products will be transformed into E_j_ coli JM101 competent cells and recombinant phages will be screened for Meg-CSF cDNA inserts as described above. Overlapping cDNA inserts from positive clones will be used to construct a full length cDNA clone for Meg-CSF. Screening of cDNA Library Expression Using Anti-peptide Antibody Polyclonal anti-peptide antiserum against the N- terminal amino acid sequence of purified Meg-CSF will be obtained by immunization of rabbits as disclosed in Cooper, H.M. et al. in Current Protocols in Molecular Biology. Ausubel, I. M. et al. eds John Wiley and Sons, N.Y. 1990. In order to make the anti-peptide antibodies, the peptide should be conju¬ gated as disclosed in Lerner R.A. Nature 299: 592-596, 1982. Alternatively, the peptide may be synthesized on a lysine core as disclosed in Tarn, J.P. Proc. Nat. Acad. Sci. USA 85: 5409- 5413, 1988. This antipeptide antibody will provide an alter¬ nate method to screen for Meg-CSF cDNA clones. Immunological screening of a lambda ZAP II cDNA library will be performed essentially as described (Huynh, T.V. et al., in Glover, D.M. (ed. ) DNA Cloning vol. 1, IRL Press, Washington, D.C. 1985; Dai, W. , Ph.D. Thesis, Purdue University, West Lafayette, Indiana, 1988). Briefly, recombinant phages grown on LB plates for 4 hr at 42"C will be overlaid with nitrocellulose filters impregnated with 2 mM isopropyl-beta-D-thiogalactopyranoside (IPTG) (Sigma Chemical Co.). After an additional 4 hr incuba¬ tion at 37°C, the filters will be removed and probed with anti- Meg-CSF serum which will have been preabsorbed with lysate from the E_j_ coli host. The membranes will be rinsed with Tris buffered saline (TBS)/Tween (Huynh, T.V. et al., supra) and probed with horseradish peroxidase-conjugated-goat-anti-rabbit (PCGAR) IgG (BioRad, Richmond, CA) . After additional TBS rinses, the nitrocellulose membranes will be placed in develop¬ ing solution (Huynh, T.V. et al., supra) for color development. Plaques yielding a positive color will recovered from plates and screened further until pure recombinant phages are ob¬ tained. Once the positive clones are confirmed, as shown by initial screening of inserts from these recombinants phage clones will be rescued into phagemid (Bluescript, Stratagene, La Jolla, CA) in the presence of "helper" phage R408 according to the protocols provided by the supplier (Stratagene) . cDNA inserts will be mapped with restriction enzymes and various restriction fragments will be subcloned into M13, M118 and mp 19 phage vectors and sequenced as outlined above. EXAMPLE 7: Expression of Biologically Active Recombinant Human Meg-CSF Due to the tremendous potential for therapeutic as well as basic research applications, the cloned Meg-CSF cDNA will be expressed immediately using both prokaryotic (e.g.. E. coli) and eukaryotic (Baculovirus) systems (e.g.. Current Protocols in Molecular Biology, John Wiley and Sons, New York 1989; see also U.S. Patent No. 4,745,051). Using either system, large amounts of the gene products of interest can be synthesized and accumulated. A chimeric gene with an appropri¬ ate "handle" peptide added to the Meg-CSF can be constructed in an appropriate vector using a DNA engineering approach. The resulting vector will be introduced into host cells for expres-r sion of the fusion protein. The fusion protein can be easily identified and purified because of the added "handle" (see below) . If the fusion protein interferes with folding and/or biological activity, the peptide "handle" can be removed by specific proteases. EXAMPLE 8: Expression of Meg-CSF in E_ Ϊ _ coli Using a Two-Plasmid System Two characteristics of E_j_ coli that make it ideally suited for an expression system for proteins are: (i) its ease of manipulation and (ii) its rapid growth in inexpensive media. Since early studies showed that the carbohydrate moieties of native Meg-CSF are not necessary for its biological activity (Murphy, M.J. et al., Acta Hematol. Jpn. 46: 1380-1396, 1983), E_i_ coli will be the first choice for expression of recombinant Meg-CSF. A prokaryotic alternative is the Baculovirus system which uses eukaryotic machinery for protein expression and post-translational modifications. Both systems are merely mentioned as non-limiting examples of the expression systems that can be used. In two-plasmid expression systems, the two plasmids are maintained within the same ∑ . coli host cell. One plasmid, the expression vector contains P τ7 upstream of the Meg-CSF gene. P T7 is the promoter element with which bacteriophage T7 RNA polymerase interacts (Tabor, S., et al., Proc. Natl. Acad. Sci. USA 12:1074-1078, 1985; Ausubel, F.M. et al., (eds) Current Protocols In Molecular Biology. John Wiley and Sons, New York, 1991). The second plasmid contains the T7 RNA polymerase gene under the control of a heat-inducible E_j_ coli promoter. Upon heat induction, the T7 RNA polymerase is produced which in turn initiates transcription of the expres¬ sion vector by recognizing the control element of P τ7 . To make the expression vector, the Bluescript plasmid containing the full length Meg-CSF cDNA insert will be linearized at the 5' end with two appropriate restriction enzymes (to cut the vector multicloning and cDNA sites respectively) which remove the cDNA insert from the 5' end up to, or slightly beyond, the Meg-CSF coding region. The amount of insert removed will depend on th availability of a restriction site in the cDNA. If no ap¬ propriate site is available, a unidirectional deletion approach 5 with Exo III nuclease will be employed (Sambrook, J. et al., supra) . The linearized vector plus most, if not all, of the coding sequence will be eluted from agarose gel with the Geneclean DNA isolation system (Bio 101, Inc. La Jolla, CA) . The eluted materials will be ligated with a short synthetic 10 oligonucleotide encoding for AspTyrLysAspAspAspAspLys residues (a specific "handle" peptide sequence for identification, purification, and cleavage purposes, Prickett, K.S. et al., Biotechniques 2:580-589, 1989) plus nucleotide sequences on both ends to generate proper ends for cloning. The ligated 15 product will be transformed into E_ t . coli XL-1 Blue competent cells and plated on LB/ampicillin plates. Plasmid DNA will be prepared (Sambrook, J. et al., supra) and the insert orienta¬ tion will be confirmed by restriction mapping and sequencing. Meg-CSF cDNA plus the fused sequence in front of its coding 20 region will then be removed from the Bluescript plasmid by ap¬ propriate restriction digestion and inserted into a pT7-7 plasmid (Ausubel, F.M. et al., supra) which has been cut with the same restriction enzymes at the cloning site. The result¬ ing plasmid will transformed into E^. coli JM105 and plated on 25 LB/ampicillin plates. Orientation of the cloned cDNA sequence with respect to P τ7 will be confirmed by restriction mapping. Plasmid pGPl-2 (obtainable from Dr. Stanley Tabor, Harvard Medical School, Boston, MA) contains a T7 RNA polymer¬ ase gene which is silent in host cells grown at 30°C and is 30 induced by raising the temperature to 42°C. Bacteria harboring the pGPl-2 plasmid will be selected on LB/Kanamycin. pT7- 7/Meg-CSF plasmid DNA will be transformed into E_j_ coli K38/pGPl-2 cells and transformants will be selected on LB plates containing ampicillin and kanamycin at 30°C. Colonies 3.5 containing two plasmids will be selected for expression of "handle" peptide/Meg-CSF fusion protein. Fresh LB/a picil- lin/Kanamycin medium will be inoculated with a single colony of bacteria containing two plasmids and bacterial cells will be grown at 30°C to an OD 590 of approximately 0.4. Expression of T7 RNA polymerase, which then activates Meg-CSF fusion gene expression, is induced by raising the temperature to 42°C. The expressed product will be analyzed biochemically by SDS-PAGE and Western blotting and functionally by in vivo and in vitro bioassays for Meg-CSF. Purification of the fusion protein can be achieved by affinity column chromatography using anti- "handle" peptide antibody (Immunex Inc., Seattle, WA) or by the procedures described above. Fused "handle" peptide can be removed by the protease enterokinase (which specifically recognizes the "handle" and cleaves at the junction of Lys/first amino acid encoded by cDNA) treatment to release the Meg-CSF protein proper. Expression of Meg-CSF Using the Baculoviral System The E_j_ coli expression system has limitations such as the lack of post-translational modification, protein processing and transfer mechanisms present in eukaryotes. There are also insolubility problems for some over-expressed foreign proteins. Baculoviruses use eukaryotic machinery for expression, and therefore, baculoviral vectors (such as those disclosed in U.S. Patent No. 4,745,051) will be used for Meg-CSF expression to circumvent potential problems. In addition to the advantages of post-translational modification and solubility of most over- expressed foreign protein, baculoviruses are noninfectious to vertebrates (Carbonell, L.F., et al., J. Virol. 56:153-160, 1985). This is particularly important when expressing poten- tially toxic proteins or oncogene products. To generate recombinant Baculoviruses, Meg-CSF cDNA will be inserted downstream of the viral polyhedrin promoter in an appropriate Baculoviral plasmid vector (e.g., pVL series; Invitrogen, Seattle, WA) as described (Ausubel, F.M. et al., supra) and the Meg-CSF cDNA will be flanked both 5' and 3' by polyhedrin gene-specific sequences. The Baculoviral plasmid DNA containing Meg-CSF cDNA will be isolated from bacterial transformants and cotransfected into Sf9 cells (Invitrogen, San Diego, CA) with wild-type viral DNA by the well known CaCl 2 - phosphate co-precipitation procedure (Rosenthal, N. et al.. Methods in Enzymol. 152:713-716. 1987). Transfected cells will be maintained in complete medium (Grace's Antheraea medium; GIBCO/BRL Life Technologies) with 10% FBS/50 microg/ml Gentamycin. Three to four days after transfection, the culture medium containing both wild type and recombinant (as a result of homologous recombination during or post co-transfection) viruses will be collected. Recombinant viruses will be puri¬ fied from wild type viruses by several rounds of plaque purifi¬ cation (Ausubel, F.M. et al., supra) . The isolated recombinant viral plaques along with agarose plugs will be placed into serum-free medium and stored at 4°C. To analyze the protein from putative recombinant viruses, Sf9 cells will be seeded at a density of 2.5 x 10 6 /25 cm 2 flask in 5 ml complete medium and maintained at 27°C for about 3 hr to allow cells to attach. Putative recombinant Baculoviruses from individual stocks in serum-free complete medium (5 x 10 3 particles) are added to the seeded cells. Three to five days post-infection, cells are gently dislodged from the flask and transferred to centrifuge tubes for centrifugation (1,000 x g, 10 min, 4°C). Since Meg-CSF is a secreted protein and should possess a leader peptide (when no "handle" peptide sequence is fused to the N-terminus) for secretion, culture supernatant (and cell lysates) will be first analyzed for the Meg-CSF antigen and activity as described below. (Alternatively, DNA encoding the secretion signal of another protein can be ligated to the Meg-CSF gene and the corresponding signal sequence can be subsequently cleaved. ) Cells infected with wild type Baculoviruses will be used as a negative control. Recombinant Baculoviral stock which shows Meg-CSF activity and gives high titers will be saved for further studies. Purification of Recombinant Meg-CSF The final purification of recombinant Meg-CSF will be performed using, for example, any or all of the following techniques: gel exclusion chromatography, reverse phase HPLC, ion exchange liquid chromatography or HPLC, hydrophobic inter¬ action chromatography, chromatofocusing, preparative polyacryl¬ amide gel electrophoresis, bioaffinity chromatography (e.g. monoclonal or other antibody columns, lectin columns and the like) as is well-knwon in the art and disclosed in Seetharam, R. et al. (eds.) in Purification and Analysis of Recombinant Proteins, Marcel Dekker, Inc., NY, 1991; Deutscher, M.P. , Methods in Enzymology Vol. 182, Guide to Protein Purification, Academic Press, NY, 1990; Ausubel, F.M. et al. (eds), Current Protocols in Molecular Biology, John Wiley and Sons, NY, 1990. The purification technique(s) to be used depend upon e.g. the cell type and cloning vector employed, as is known to those of ordinary skill in the art. For example, HPLC may be employed as used in Mochizuki, D.Y. et al. J. Immunol. JL3£:3706-3709, 1986 for the purification of GM-CSF obtained from yeast cells, chromatofocusing as disclosed in Zsebo, K.M. et al. J. Biol. Chem. 261:5858-5865, 1986 for the purification of yeast-secret¬ ed concensus interferon, anion exchange and gel filtration as disclosed by Janoff A. et al. (Am. Rev. Resp. Dis. 133:353-356, 1986) to purify yeast-derived alpha : proteinase inhibitor. In addition, Baculovirus-.derived hMeg-CSF may be purified using techiques disclosd in Summers, M.D. et al. "A Manual of Methods for Baculovirus Vectors and Insect Cell Cultures", Bulliten 1555, Texas Agricultural Experimental Station, College Station, TX, 1987. The glycosylation of native hMeg-CSF may have a significant effect on its behavior, especially on the ion exchange columns. Therefore, it may be necessary to use alternative methods to purify recombinant materials expressed in £j_ coli, as disclosed in Seetharam, R. et al. (supra) , which would be unglycosylated. These include expressing recombinant hMeg-CSF as a fusion protein which can be isolated by an affinity step using available antibodies (Immunex Inc., Seat- tie, WA) to the "handle" peptide described above or other tech¬ niques, as disclosed in Seetharam, R. et al. (supra) , such as ultrafiltration, or solution based chromatofocusing described above and apparent to those skilled in the art. All or part of the purification scheme outlined in prior examples is anticipated to be useful in purification of recombinant hMeg-CSF. Example 9: Use of hMeg-CSF in Elucidating Mechanisms of Platelet Production The homogeneous hMeg-CSF of the present invention will be employed in experiments using either human or murine bone marrow to elucidate the role of Meg-CSF in megakaryocytopoiesis. The following questions will be asked: Does Meg-CSF cause proliferation of bone marrow progenitors so that more megakaryocytes are produced? Does it cause differen¬ tiation of progenitors into megakaryocytes, thereby affecting the number of platelets and/or the rate at which they are formed? Does it interact with other cytokines to increase the proliferation and/or differentiation of megakaryocytes? Experiments to examine these questions would use bone marrow in CFU-Meg assays. Meg-CSF will be added either simul¬ taneously or sequentially with other factors (before or after) to determine which factors promote stem cell differentiation down the megakaryocytic lineage versus those that drive CFU-meg proliferation versus those that increase maturation to platelet formation and release. The number of CFU-Meg that are formed will be determined. It will also be determined whether these are increased over the control cultures. It will also be determined if the number of megakaryocytes are increased over those formed with other cytokines. An alternative to the CFU- Meg assays will be to use liquid cultures of medium and fetal calf serum to grow the bone marrow and count the number of megakaryocytes that are formed. It will also be determined whether Meg-CSF produced according to the present invention synergizes with other cytokines such as IL-6, GM-CSF, IL-3, EPO, IL-11, SCF, LIF, activins/inhibins, or TPO/TSF to produce even more megakaryocytes (in the CFU-Meg assay the number of megakaryo¬ cyte colonies and/or number of megakaryocyte cells per colony can be measured) than each cytokine alone. Finally, the maturity of the resulting megakaryocytes by their size, inten¬ sity for staining for specific platelet proteins (such as acetylcholmesterase for mice; von Willebrand's factor, gpIIb/IIIa, factor VIII for human megakaryocytes) and ploidy measured by specific DNA staining (e.g. Hoescht 33258 stain) and flow cytometry will be determined. In addition, it will be determined which factors and in what combination with Meg-CSF gives the optimal megakaryo¬ cyte colony formation. Alternatively, it will be determined which combination gives the maximum number of megakaryocytes in liquid culture. Example 10: Use of Additional Screening Tools in Identifying hMeg-CSF DNA To facilitate the molecular cloning/characterization and eventual cDNA isolation coding for hMeg-CSF, oligodeoxynucleotides corresponding to three stretches (oligo 1; 5'-5746-5769-3'; oligo 2; 5'-5922-5965-3' ; and oligo 3; 5'- 7446-7417-3') according to the DNA sequence disclosed in PCT Application No. WO 91/02001 published Feb. 21, 1991, were e - ployed in a PCR amplification study. The purpose was to generate DNA strands that could be used as probes to facilitate the isolation of the hMeg-CSF gene(s) of the present invention, either via hybridization under relatively low stringency conditions (if several genes exist which code for more than one hMeg-CSF) or via high-stringency hybridization to a gene that indeed encodes a hMeg-CSF protein and in particular either or both species of hMeg-CSF of the present invention, which is no believed to be the same as any gene that PCT WO 91/02001 may b directed to. Genomic fragments were amplified from a human placenta genomic DNA library according to a protocol provided by the supplier (Perkin Elmer Cetus, Inc, Norwalk, Connecti¬ cut) . One μg of the DNA was used in a final reaction volume o 100 μl using primer pairs as specified in Fig. 12. PCR was performed at 94'C for 1 min. , 45'C for 2 min. and 72'C for 3 min. for 3 cycles and 94 * C for 1 min., 50'C for 2 min. and 72'C for 3 min. for 40 additional cycles. The amplified products were analyzed on a 1% agarose gel as shown in Fig. 12. Using oligos No. 1 and 3, agarose gel electrophoresis showed that tw specific products were amplified with molecular sizes of 1.65 kbp and 300 bp, as shown in Fig. 12. To confirm the specific- ity, the two fragments were eluted from the agarose gel and subjected to a second round of PCR amplification using oligos and 3. As shown in the same figure, the larger fragment was amplified as predicted in terms of its specificity and molecu¬ lar size. However, the smaller one (300 bp) was not amplifi- able with oligos 2 and 3 as primers, although it is specifical ly amplified by oligos- 1 and 3 (Fig. 12, lane 9). It appears therefore that: (1) Two or more Meg-CSF genes could exist either as alleles or as different genes on the haploid chromosome of which the Meg-CSF gene giving rise to the 300 bp fragment is deprived of at least part of oligo 2 sequences. In that case, WO 91/02001 may be directed to a portion of one but not the other; (2) WO 91/02001 is directed to DNA sequences which encode yet another different species of hMeg-CSF. In that case, the DNA amplified (i.e., the 300 bp fragment) as de¬ scribed in this Example 12 would be useful as a probe for identifying and isolating the gene that contains it (which is different from any DNA isolated by WO 91/2001), and the 1.65 bp fragment is anticipated to be useful for identifying the present hMeg-CSF by hybridization under low-stringency condi¬ tions.

Claims: WHAT IS CLAIMED IS: 1. An isolated, purified human megakaryocyte colon stimulating factor, said factor having the following proper- ties: a) being free of detectable EPO and GM-CSF activities; b) being homogeneous as determined by exis- tence of a single amino terminal amino acid sequence and the ability to migrate as a single band after electrophoresis on sodium dodecyl sulfate polyacrylamide gels; and c) having the ability to induce the formation of megakaryocyte colony-forming units in a murine fibrin clot assay in vitro. 2. The factor of claim 1 further having the ability to stimulate the production of platelets when administered to a mammal. 3. The factor of claim 2 having a molecular weight ranging between about 50,000 daltons and 70,000 daltons when in glycosylated and sialyated form. 4. The factor of claim 3 having an isoelectric point ranging between about 7.2 and about 7.4 as determined by isoelectric focusing when in glycosylated and sialyated form. 5. The factor of claim 4 having the ability to stimulate the formation of at least about 4,000 megakaryocyte colony forming units per milligram protein in a murine fibrin clot assay m vitro. 6. The factor of claim 5 comprising a weakly basic protein. 7. An isolated, purified human megakaryocyte colony stimulating factor preparation comprising at least about 90% protein said preparation being characterized as: a) being free of detectable EPO and GM-CSF activities, b) having a pi between about 7.2 and 7.4 when in glycosylated and sialyated form, and c) having the ability to induce the formation of megakaryocyte colony forming units in a murine fibrin clot assay in vitro. 8. An isolated, virtually pure human Meg-CSF protein fraction, said protein fraction having, when in glyco- sylated and sialyated form: a) a molecular weight within at least one range selected from the group consisting of (i) either about 24,000 daltons and about 35,000 daltons and (ii) about 50,000 daltons and about 70,000 daltons as determined by SDS-PAGE; and b) an isoelectric point ranging between about 7.2 and 7.4 as determined by isoelectric focusing, said fraction containing at least about 90% protein and being free of detectable EPO and GM-CSF activity and having the ability to induce the formation of megakaryocyte colony forming units in a murine fibrin clot assay in vitro. 9. An isolated, purified polypeptide having human megakaryocyte colony stimulating factor activity and a single amino-terminal and comprising at its free amino terminus the amino acid sequence X-Asp-Pro-Val-Glu-Ser-Pro-Val-Pro-Y. 10. The polypeptide of claim 9 consisting essential- ly of a biologically active fragment of said polypeptide. 11. An isolated, purified recombinant human mega- karyocyte colony stimulating factor. 12. A pharmaceutical formulation for administration to a mammal suffering from a disease related to the production of platelets comprising an isolated, purified human megakaryo- cyte colony stimulating factor protein, said protein having the following properties: a) being free of detectable EPO and GM-CSF activities, b) being homogeneous as determined by having a single amino terminal amino acid sequence and migrating as a single band after electrophoresis in sodium dodecyl εulfate polyacrylamide gels; and c) having the ability to stimulate the forma- tion of megakaryocyte colony forming units in a murine fibrin clot assay in vitro. 13. A pharmaceutical formulation for administration to a mammal suffering from a disease related to the production of platelets comprising an isolated, purified polypeptide having human megakaryocyte colony stimulating factor activity and comprising at its free amino terminus the amino acid sequence X-Asp-Pro-Val-Glu-Ser-Pro-Val-Pro-Y. 14. A method for isolating a human Meg-CSF protein fraction said fraction having a protein content of at least 90% and being free of detectable EPO and GM-CSF activity, said method comprising the steps of: a) concentrating urine from aplastic anemia patients; b) desalting the concentrated urine; c) removing non-ionic contaminants contained in the desalted concentrated urine by applying it to an ion exchange support and eluting from said support an impure protein fraction containing human Meg-CSF; d) applying the impure protein fraction to a preparative polyacrylamide electrophoresis gel under non- denaturing conditions and isolating from said gel a substan- tially pure Meg-CSF fraction; e) subjecting said substantially pure Meg-CSF fraction to a further purification step selected from the group consisting of i) conventional ion-exchange chromatogra- phy; ii) ion-exchange chromatography using a cation-exchange high performance liq- uid chromatography column; and ϋ) gel electrofocusing at a pH gradient between about 3.5 and about 10 and recovering a further purified Meg-CSF fraction; f) subjecting said further purified fraction to reverse phase high performance liquid chromatography and recovering a virtually pure hMeg-CSF fraction containing at least 90% protein and being free of detectable EPO and GM-CSF activity. 15. The method of claim 14 further comprising dialyzing said impure protein fraction obtained in step (c) prior to step (d) . 16. The method of claim 15 further comprising lyophilizing said dialyzed protein fraction, and subjecting the lyophilized fraction after resuspension to step (d) . 17. The method of claim 14 wherein said ion exchange support in step (c) is selected from the group consisting of DEAE-cellulose and CM-Sepharose columns. 18. The method of claim 14 wherein said ion exchange chromatography in step (e)(i) comprises a MonoP chromatographic support. 19. The method of claim 14 wherein said cation exchange high performance liquid chromatography comprises is a WCX polyaspartic acid chromatographic support. 20. The method of claim 14 wherein said reverse phase high performance liquid chromatography is performed using a C18 column. 21. The method of claim 14 wherein said hMeg-CSF fraction of step(f) is dialyzed. 22. The method of claim 21 wherein the product of the dialysis step is lyophilized. 23. The method of claim 14 wherein said step (a) comprises ' ultrafiltration using a YM10 membrane. 24. The method of claim 14 wherein said desalting step comprises gel filtration using a G-25 gel. 25. The method of claim 14 wherein said preparative polyacrylamide gel is a 5% polyacrylamide gel. 26. A method for isolating a homogeneous human megakaryocyte stimulating factor (hMeg-CSF) protein comprising the steps of: a) concentrating urine containing hMeg-CSF by ultrafiltration; b) desalting the concentrated urine by gel filtration; c) removing non-ionic contaminants contained in the desalted concentrated urine by applying it to an ion exchange chromatographic support and eluting from said support an impure protein fraction containing human Meg-CSF; d) applying the impure protein fraction to a preparative polyacrylamide electrophoresis gel under non- denaturing conditions and isolating from said gel a substan- tially pure Meg-CSF fraction; e) further purifying said substantially pure hMeg-CSF factor by chromatofocusing; f) subjecting said further purified fraction to reverse-phase high-performance liquid chromatography and recovering an isolated hMeg-CSF fraction; and g) subjecting said isolated hMeg-CSF fraction to cation high performance liquid chromatography and recovering a homogeneous hMeg-CSF protein. 27. The method of claim 26 further comprising dialyzing said protein fraction obtained in step (c) prior to step (d) . 28. The method of claim 27 further comprising lyophilizing said dialyzed protein fraction. 29. The method of claim 26 wherein said ion exchange support in step '(c) is selected from the group consisting of DEAE-cellulose and CM-Sepharose. 30. The method of claim 26 wherein said ion exchange chromatography in step (e) (i) comprises a MonoP chromatography column. 31. The method of claim 26 wherein said preparative polyacrylamide gel is a 5% polyacrylamide gel. 32. The method of claim 26 wherein said chromatofocusing is performed using a MonoP HR 5/20 column. 33. The method of claim 32 wherein said chromatofo- cusing is performed using a polybuffer gradient at a pH ranging between about pH 8 and about pH 6. 34. The method of claim 26 wherein said cation exchange high-performance liquid chromatography is performed using a WCX polyaspartic acid column. 35. The method of claim 34 wherein the WCX high- performance liquid chromatography is performed at pH 6.42- 36. A method for treating a mammal suffering from a disease related to the production of platelets comprising administering to a mammal in need of such treatment an effec- tive amount of an isolated, purified human megakaryocyte colony stimulating factor, said factor having the following proper- ties: a) being free of detectable EPO and GM-CSF activities; b) being homogeneous as determined by exis- tence of a single amino terminal amino acid sequence and migration as a single band after electrophoresis on sodium dodecyl sulfate polyacrylamide gels; and c) having the ability to induce the formation of megakaryocyte colony-forming units in a murine fibrin clot assay in vitro. 37. A method for treating a mammal suffering from a disease related to the production of platelets comprising administering to a mammal in need of such treatment an effec- tive amount of an isolated, virtually pure human Meg-CSF protein fraction having, when in glycosylated and sialyated form: a) a molecular weight within the range select- ed from the group consisting of about 24,000 daltons and about 35,000 daltons and about 50,000 daltons and about 70,000 daltons and combinations thereof, said molecular weights being determined by SDS-PAGE; and b) an isoelectric point ranging between about 7.2 and 7.4 as determined by isoelectric focusing, said fraction containing about 90% protein and being free of detectable EPO and GM-CSF activity and having the ability to induce the formation of megakaryocyte colony forming units in a murine fibrin clot assay in vitro. 38. An isolated, purified human megakaryocyte colony stimulating factor, said factor having the following properties: a) being free of detectable EPO and GM-CSF activities; b) being homogeneous as determined by migra- tion as at least one of two bands after electrophoresis on sodium dodecyl sulfate polyacrylamide gel, one of said bands corresponding to a species of said factor within the molecular weight range of about 50 to about 70 kD and the other corre- sponding to a species within the molecular weight range of about 24 to about 35 kD when in glycosylated and sialyated form. c) having the ability to induce the formation of megakaryocyte colony-forming units in a murine fibrin clot assay in vitro. 39. The factor of claim 1, said factor being free of ∑ 0.05 u/ml of EPO; ∑ 12.5 u/ml of GM-CSF; ∑ 6 u/ml of IL-3; ∑ 5 u/ml of IL-9; ∑ 10 u/ml of each of G-CSF and M-CSF; ∑ 62.5 pg/ml of IL-6; and 31.3 pg/ml of IL-1 alpha. 40. The preparation of claim 7 having a molecular weight within at least one range selected from the group consisting of (i) either about 24,000 daltons and about 35,000 daltons and (ii) about 50,000 daltons and about 70,000 daltons as determined by SDS-PAGE. 41. The factor of claim 1 further having the proper- ty of experiencing a shift in the bulk of its MEG-CSF activity from a molecular weight range of about 50 to about 70 kD upon SDS-PAGE under non-reducing conditions to a molecular weight range of about 24 to about 35 kD upon SDS-PAGE under reducing conditions. 42. An oligonucleotide having the formula selected from the group consisting of GAC CCN GTN GAA TCN CC, GAT CCN GTN GAA TCN CC, GAT CCN GTN GAG TCN CC, and GAT CNN GTN GAG TCN CC, wherein N denotes all four nucleotides. 43. An oligonucleotide having the formula selected from the group consisting of TCI CCT GTN CCN GAG and TCI CNN GTN CCN GAA, wherein N denotes any one of nucleotides C, T, A, G and I denotes inosine. 44. An oligonucleotide having the formula selected from the group consisting of GCN CCN GTN GAY TGZ CC, wherein W is T or C, Y is A or G, Z is C or T, and N denotes any one of the nucleotides C, T, A or G. 45. A method for isolating a gene encoding hMeg-CSF comprising probing human genomic DNA with at least one oligonu- cleotide according to claims 42, 43 or 44 and selecting human genomic DNA sequences hybridizing with said oligonucleotide under stringent conditions. 46. An isolated purified 300 bp fragment of genomic human DNA that has the property of being amplified by oligonu- cleotide sequences TCT CTC TCA CCA AGT GGC TTT GTC; and TGA GAG TAT TAG CCC TGT CGT GAA CAG TAT but not by oligonucleotide sequence C ATG GAG TGC TGC CCT GAT TTC AAG AGA GTC TGC ACT GCG GGTA.

Inventors: DAI WEI, ERICKSON-MILLER CONNIE L, KRUTZSCH HENRY, LIOTTA LANCE A, MURPHY MARTIN J, PARCHMENT RALPH E, ZHANG ZHAO-GENG, DAI, WEI, ERICKSON-MILLER, CONNIE, L., KRUTZSCH, HENRY, LIOTTA, LANCE, A., MURPHY, MARTIN, J., PARCHMENT, RALPH, E., ZHANG, ZHAO-GENG
Applicants: HIPPLE CANCER RES CENTER, HIPPLE CANCER RESEARCH CENTER
Application year: 1991
Main classification: C07K17
IPC codes: A61K38, A61P7, C07K1, C07K14, C12P21