CELL SEPARATION COMPOSITION, KIT AND METHOD

Abstract: Disclosed are a kit, composition and method for cell separation. The kit includes a centrifugable container and an organosilanized silica particle-based cell separation suspension suitable for density gradient separation, containing a polylactam and sterilized by treatment with ionizing radiation. The composition includes a silanized silica particle-based suspension for cell separation which contains at least 0.05 % of a polylactam, and preferably treated by ionizing radiation. Also disclosed is a method of isolating rare blood cells from a blood cell mixture.

Description: CELL SEPARATION COMPOSITION, KIT AND METHOD Field of the Invention The present invention relates to compositions, kits and methods for cell separation. Background of the Invention Separation of cells and cellular components, long an important tool in basic research and diagnostic applications, is becoming increasingly important in the clinical setting. The use of cell transplantation memods, such as bone marrow transplantation, in human therapeutics has necessitated cell separation procedures that are not only rapid and reproducible but which also produce viable, safe, non-toxic cell composition. It is also desirable that such isolation procedures are suitable for isolating cells that are relatively low in abundance. For example, following chemotherapy for certain types of cancer, many patients now receive "stem cell transplants" which consist of an enriched fraction of hematopoietic progenitor cells isolated from various tissues, such as bone marrow, peripheral blood or cord blood. Such progenitor cells constitute only about 1 % of the total number of cells present in these tissues. Density gradient centrifugation is a popular technique for separating and isolating cells and cellular components. This method exploits the phenomenon that cells partition in a defined density medium according to their buoyant densities. Such a medium may be a solution or a suspension of microparticles. Examples of media commonly used in density gradient separation include sucrose, dextran, bovine serum albumin (BSA), FICOLL diatrizoate (Pharmacia), FICOLL metrizoate (Nycomed), PERCOLL (Pharmacia), metrizamide, and heavy salts such as cesium chloride. Exposure of cells to many of these media produces impaired biological function of the cells and/or contamination of the preparation by toxic components. For example, BSA and FICOLL are known to cause unwanted cell aggregation at physiological pH. In addition, these media are not generally amenable to sterilization by autoclave or irradiation in "final form" - that is, in a concentration, ionic solution and container that is ready to use in isolating a particular cell type. One cell separation medium that has been widely used in preparation of blood cell fractions is PERCOLL (a registered trademark of Pharmacia Fine Chemicals). PERCOLL is a colloidal silica particle treated by a curing process to form a polyvinylpyrrolidone coating on each particle. While PERCOLL is fairly stable at physiological pH, there are some limitations to its general usefulness in isolating cells for therapeutic purposes. For example, the medium is difficult to sterilize, since it is not stable to autoclaving or ionizing irradiation after it is diluted in a physiological salt solution. These properties limit the usefulness ofthe product for clinical applications involving re-introduction of separated cells into humans or anywhere else mat finally sterilized material is required. U.S. Patent 4,927,749 provides an organosilanized colloidal silica (OCS) particle (OCSP) preparation for density gradient separation that overcomes some of the problems discussed above. While it is stable to sterilization by heat and ionizing irradiation when diluted in a physiological salt solution, when sterilized by ionizing radiation, OCS particle preparations are toxic to certain rare cell types, such as dendritic cells, natural killer cells, natural suppressor cells, cytotoxic T cells, and in particular, hematopoietic progenitor (CD34 + ) cells. Since final sterilization by ionizing radiation may be preferred for certain applications, particularly those involving containment ofthe material in plastic consumable vessels, the use of this material for clinical applications may be limited. Moreover, it has been found that the material induces clumping or aggregation of cells, resulting in significantly reduced yields of such rare cells in functional form. The present invention provides compositions and methods that are particularly suitable for isolation of "rare" cell fractions--^. , cell types that constitute less than about 1 % of total cells in a cell population. In particular, the invention includes a colloidal silica cell separation media that improves the yield and functional potential of cells by reducing cell aggregation and cell toxicity. It is the discovery of the present invention that inclusion of a polylactam, such as the gamma polylactam polyvinylpyrrolidone (PVP), in the solution in which colloidal silica density gradient material is suspended markedly reduces aggregation of rare blood cells, result¬ ing in improved yields. It is a further discovery of the present invention that addition of the polylactam to the medium prevents loss of clonogenic potential during density gradient isolation of certain hematopoietic progenitor cells by reducing the irradiation-induced toxicity of OCS solutions. The invention also provides methods for isolation of specific cell types that are important in diagnostic and therapeutic methods-including, for example, hematopoietic progenitor (CD34 + ) cells isolated from bone marrow, selected tumor cells, dendritic cells, natural killer (NK) cells, cytotoxic T-lymphocytes (CTL) and natural suppressor cells. The invention also provides for the depletion of cell types that may interfere with a particular clinical outcome, such as CTL. Summary of the Invention In one aspect, the invention includes compositions for use in cell separation consisting of a silanized colloidal silica particle-based cell separation medium containing at least about 0.05% polylactam, such as polyvinylpyrrolidone (PVP), present at a concentration between about 0.1 and about 10 percent. In a preferred embodiment, the separation medium is composed of a suspension of organosiianized colloidal silica particles. An advantage of this composition is that it can be sterilized, as by autoclaving or by ionizing radiation, without imparting toxic effects on the cells subsequently separated therein. While particles comprising the media of the invention can range from about 0.003 to 50 microns, colloidal silica particles are usually on the average of 10-50 nm in diameter. Compositions of the invention are particularly suited for use in isolating a selected rare blood cell from a cell mixture. According to methods described herein, in such cases, the medium is prepared such that the particle suspension has a specific density within about 0.0005 gr/ml of said selected cell. Exemplary rare cell types include hematopoietic progenitor CD34 + cells, dendritic cells, natural killer cells, natural suppressor cells, cytotoxic T cells, tumor cells and nucleated fetal cells. Hematopoietic progenitor CD34 + cells may be isolated from peripheral blood mononuclear cells, in which case the preferred organosiianized colloidal silica particle suspension has a specific density of 1.0605 gr/ml. When hematopoietic progenitor CD34 + cells are isolated from bone marrow cells, the organosiianized colloidal silica particle suspension preferably has a specific density of 1.0685 gr/ml. Dendritic cells are also isolated using compositions described above. In this case, suspensions having densities of 1.0770 gr/ml, 1.0720 gr/ml, 1.0650 gr/ml, 1.0610 gr/ml, and 1.0565 gr/ml may be utilized. The invention also includes a kit for cell separation. The kit consists of a centrifugable container, and a silanized colloidal silica panicle-based cell separation suspension as described above. Also included in the present invention is a method of sterilizing a silanized silica particle- based cell separation medium, where the separation medium is subjected to autoclaving or ionizing radiation in the presence of at least about 0.05 percent, and preferably between about 0.1 and 10 percent, polylactam. More generally, the invention includes a method of isolating a selected rare blood cell from a cell mixture where the mixture is first layered onto a silanized colloidal silica particle- based cell separation medium having a specific density within about 0.0005 gr/ml of the selected cell. The mixture is then is centrifuged. According to a preferred embodiment, the centrifugation is carried out in a so-called "cell-trap" tube which contains: (i) side walls and a closed bottom, and (ii) a constriction member disposed within the tube, where the constriction member is positioned and constructed to retain fluid in the bottom portion of the tube below the constriction member, when the tube is inverted. Also, in this configuration, prior to centrifugation, the tube is filled with the separation medium to a level extending above said constriction member to a level above an opening formed by said constriction member, such that cells which are captured at an interface between the cell-separation medium and a lower- density medium, after centrifugation, are discharged with the lower-density medium when the tube is inverted. The tube may also include a closed top, a first port in top through which fluid material can be introduced into the tube, a second port in the top, and a closed fluid channel communicating the second port with the bottom of the tube below the constriction member. This configuration can be used to isolated functional hematopoietic progenitor cells at separation medium concentrations of 1.0605 gr/ml (from peripheral blood cell mixtures) and 1.0685 gr/ml (from bone marrow). Tumor cells are also isolated by this method using a cell separation medium of a specific density selected from the range 1.0490-1.0580 gr/ml. Likewise, dendritic cells can be isolated using separation media having specific densities of 1.0770 gr/ml, 1.0720 gr/ml, 1.0650 gr/ml, 1.0610 gr/ml, and 1.0565 gr/ml. Cytotoxic T lymphocytes may be isolated using a specific density of 1.0720 gr/ml, 1.0610 gr/ml or 1.0565 gr/ml. The foregoing separation methods may be enhanced by adding a suspension which includes a silanized silica particle to which is attached a cell antigen-specific binding molecule. Such addition may be used to deplete unwanted cell types from a preparation, such as T-cell lymphocytes. Brief Description of the Figures FIG. 1 shows recovery of functional hematopoietic progenitor cells (colony forming units, CFU) after incubation with gamma-irradiated 0.5% PVP in PBS, gamma-irradiated OCS particle-based density gradient material (OCSP), or gamma-irradiated OCS particle-based density gradient material supplemented with 0.5% PVP (OCSP+0.5% PVP); FIG. 2 shows the effect of various concentrations of PVP present during sterilization of OCS particles (OCSP) on gamma-irradiation-induced restriction of hematopoietic progenitor cell function (CFU); FIG. 3 shows the distribution of PBPC cells in interface and pellet fractions of density gradients formed from colloidal silica particles in the absence (OCSP) and presence of 0.5% PVP (OCSP+PVP); FIG. 4 shows the distribution of hematopoietic progenitor cells (CD34 + ) cells in interface and pellet fractions of density gradients formed from colloidal silica particles (OCSP) in the absence and presence of 0.5% PVP (OCSP + PVP); FIG. 5 shows a comparison of non-specific depletion of PBMC associated with DACS beads coated with two types of GAM-IgG preparations; FIG. 6 shows non-specific loss of CD34 + cells with DACS coated with intact GAM-IgG or F(ab)2 GAM-IgG; FIG. 7 shows non-specific loss of CD34 + cells with DACS beads coated with intact GAM-IgG in either a random or a site-specific orientation; FIG. 8 shows percent recovery of total cells, CD34 + cells, CTL's and red blood cells (RBC) from the combined interface + pellet after centrifugation of bone marrow cells on an OCS gradient; FIG. 9 shows the percent recovery of total cells, CD34 + cells, CTL's and red blood cells (RBC) from the interface of the gradient following centrifugation; FIG. 10 shows distribution of natural suppressor cell activity in different density fractions; FIG. 11 shows distribution of natural killer cells in different density fractions; FIG. 12 shows results of experiments in which peripheral blood buffy coat samples were depleted of human B lymphoma cells by DACS; FIG. 13 shows recovery of CD34 + cells in the depleted peripheral blood buffy coat samples of FIG. 12; FIG. 14 shows a sterilized centrifuge kit assembled in accordance with the invention; and FIG. 15 shows a closed system centrifugation device suitable for inclusion in a kit of die invention. Detailed Description of the Invention The invention encompasses cell density separation media suitable for separation of cells, and particularly for separation or isolation of cells to be transplanted into humans. The invention also includes meϋ ods for isolating specific cell types, preferably using the new medium composition. As described in the sections that follow, the media of the invention have been developed in view of the applicants' discovery that cell toxicity is significantly reduced when a solution of a gamma polylactam such as polyvinylpyrrolidone is added to a suspension of organo¬ siianized colloidal silica (OCS) particles used in density gradient separation of the cells. In particular, inclusion of the polylactam prevents loss of clonogenic potential in rare populations of hematopoietic progenitor cells mat occurs when such cells are suspended in a suspension of sterile silanized silica particles. In addition, the present invention includes d e discovery that inclusion of polylactams such as polyvinylpyrrolidone in the density gradient medium also reduces aggregation and loss of clonogenic potential of hematopoietic progenitor cells that occurs wiϋ out ionizing radiation. I. Definitions "Colloidal silica" refers to an aqueous suspension of colloidal particles formed by polymerization of monosilicic acid from Si0 2 dissolved in water. "Silanized silica particles" refers to a particulate silica composition to which is covalently linked an silane coating. "Organosiianized colloidal silica (OCS) particles" refers to a colloidal silica composition to which is covalently attached an organosilane coating. U.S. Patent 4,927,749 describes how to prepare such a composition. "Rare cell populations" refers to cells that constitute less than about 1% ofthe total white blood cell (WBC) count in the blood of a healthy individual. Examples of rare blood cells include CD34 + hematopoietic progenitor cells, which constitute about 1 % of WBC's, natural killer cells, Natural suppressor cells, dendritic cells, and cytotoxic T cells. Also included in this definition are cells from exogenous sources, including circulatory fetal cells in maternal blood. "Cell toxicity", "toxic to cells", and similar phrases herein refer to any diminution in cell viability or biological function that is measurable in a cell population. Wiϋh reference to the hematopoietic progenitor cells described herein, the term most commonly refers to diminution in clonogenic potential, as evidenced by reduced yield of cells capable of forming hematopoietic colonies (CFU). A "polylactam" is a polymer containing pendant lactam (cyclic amide) groups attached to the polymer backbone. The polymer may be a homopolymer or a co-polymer. The pendant lactam groups will generally have ring sizes containing from 3-6 carbon atoms. One preferred type of polylactam for use in the present invention is a polyvinyl lactam, such as polyvinylpyr¬ rolidone, which has a 4 carbon ring. "Dendritic cells", or "DC" are immune cells that are negative for expression of CD3, CD4, CD8, CD14, CD16 and CD20, and positive for expression of HLA-DR (i.e. , class II MHC). Dendritic cells exhibit a distinctive moφhology-that is, ϋ ey are large veiled cells which extend dendrites when cultured in vitro. A "tumor cell antigen" is a molecule, generally a protein, that is exposed on the surface of a particular tumor cell. Examples of tumor cell antigens include CD9, CD10, CD19 and CD20 present on B cell lymphoma cells; tumor cell antigens that are specific for certain breast tumors include Her2/Neu and estrogen receptors. In the context of the present invention, the term "isotonic" means having an osmolality that is within the range tolerated by me cell, usually about 280-340 mOsm/kg H 2 0, and preferably, for human cells, about 280 mOsm/kg H 2 0, depending on the cell type and source. II. Cell Separation Medium This section describes various cell separation media that can be used to isolate cells from cell mixture. Preferred methods of cell separation described in the sections which follow include use of a cell separation material, such as a density gradient material, having a specific gravity between 1.0000 gr/ml and 2.0000 gr/ml, preferably between 1.0300 gr/ml and 1.2000 gr/ml, that is accurate within ± 0.0005 gr/ml, preferably ± 0.0002 gr/ml of the specific gravity of the desired cell. A. Density Gradient Media Preferred density gradient media are colloidal silica suspensions having particles ranging from 0.002 to 50 microns. One useful medium is PERCOLL, a colloidal silica particle suspension which is not silanized and which is available from Pharmacia Fine Chemicals (Piscataway, N.J.). More preferred is an organosiianized colloidal silica particle suspension as described in U.S. Patent 4,927,749, and treated with PVP, as described below. According to an important feature of die invention, the density gradient solution should be prepared and adjusted to a pre-determined density. In addition, ϋhe osmolality should be adjusted in the range of 280 to 320 mOsm/kg H 2 0 to preserve cell integrity, and the pH should preferably be in the range from 6.8 to 7.8, and most preferably pH 7.4, for maintaining a physiologically isotonic density gradient, prior to use. The osmolality and pH may vary depending upon the particular conditions under which the density gradient separation memod is performed. For example, the temperature at which die samples are maintained or centrifuged may necessitate modifications to die osmolality and/or pH of die density gradient material, in order to maintain fhe appropriate density. Such modifications of fhe osmolality and pH will be apparent to those skilled in die art. A preferred density gradient material for use in separating cells in accordance wiϋh the present invention is an organosiianized colloidal silica (OCS) particle suspension, such as are disclosed in U.S. Patent 4,927,749 to Dorn, incorporated herein by reference. In a preferred embodiment, the OCS density gradient material is diluted to an appropriate specific density in a physiological salt solution supplemented with polyvinylpyrrolidone (PVP) such as PVP-10 available from Sigma Chemical Co. (St. Louis, MO). According to an important feature, as discussed below, it has been found mat inclusion of at least 0.5% PVP improves the yield of functional rare blood cell types. In addition, such a concentration of PVP facilitates sterilization of the OCS density gradient material by ionizing radiation, such as E- beam or gamma irradiation. In the absence of PVP, cells exposed to the medium experience loss of functional capacity— in ie case of CD34 + cells, evidenced by loss of ability to form colonies (reduction of CFU). Generally, die amount of PVP to be added to an OCS cell separation suspension will depend on d e subsequent purpose and treatment of ie preparation. For preserving functional activity of cells exposed to the OCS preparation for relatively short periods of time, a concentration of 0.5% (wt/wt) PVP generally suffices. For longer exposures and exposure to ionizing radiation, higher concentrations of PVP may be required— as high as about 3-8% (wt/wt). B. Silanized Silica Particles The mediods and compositions described herein are applicable to a number of uses of silanized silica particles in conjunction with cell separation. The particles can be silanized by any of a number of methods known in die art. U.S. Patent 4,927,749, incoφorated herein by reference, describes preparation of OCS particles that are particularly useful in preparing die compositions used in die present invention. In one embodiment, OCS particles ranging in size from 3-22 nm, and particularly from 13-18 nm, form a stable suspension mat acts as a density gradient medium for separating heterogeneous mixtures of cells on die basis of buoyant density. In addition to forming density gradient materials, silanized silica particles can also be used in od er aspects of cell separation. For example, particles used in certain positive or negative selection steps may be treated in accordance with die methods described herein. Such particles are typically silanized silica microspheres or beads having diameters ranging up to about 50 microns, and conventionally, within d e range of about 1-5 microns. Such particles are silanized according to methods well known in the art, for example, using 3-aminopropyltrieth- oxysilane, (3-iodopropyl) trimedioxysilane, [l-9trimethoxysilyl)-2(m-(or p) chloromethyl)- phenyl] ethane, or 3-glycidyl-oxypropyltrimethoxysilane (GPMS). Typically, mese particles have as covalent attachments specific antigen binding molecules, such as antibodies, lectins or other cell binding agents. When added to a cell mixture, the microspheres bind to cells having a specific target antigen or antigens, and modify (increase or decrease) die density of the cells. Polyvinylpyrrolidone (PVP) has been added to suspensions of non-silanized colloidal silica particles (Pertoft, et al., Exp. Cell Res. 46: 621-623 (1966); Pertoft, et al., Exp. Cell Res. 50: 355-368 (1968); Wolff, et al., J. Cell Biol., 55: 579-585 (1972); Pertoft, et al., Exp. Cell Res., 110: 449-457 (1977)). However, free PVP in solution has been reported to have undesirable effects on some cell types (Pertoft, et al., Exp. Cell Res. 50: 355-368 (1968)). This lead to development of a diermal curing process mat produces a PVP coating adsorbed to die colloidal silica particle, and which is substantially free of free PVP. (Pertoft, et al., Anal. Biochem. 88: 271-282 (1978)). This composition is commercially available as PERCOLL. However, PERCOLL cannot be sterilized by conventional means (autoclaving or ionizing radiation) when diluted in a physiological salt solution at a final concentration suitable for cell separation. This limits its usefulness in applications that require a finally sterilized density gradient material in a physiological medium. As noted above, a colloidal silica particle-based preparation having a covalent organosilane coating has been found to have utility in separating various blood cells (U.S. Patent 4,927,749). The organosilane coating reduces cell toxicity and eliminates aggregation of me colloidal silica particles themselves in die presence of physiological salt and protein. However organosilane-coated colloidal silica (OCS) particles have certain disadvantages when used to separate or purify specific rare blood cells from blood and ofher sources. Use of such particles may result in reduced recovery of such rare cells. This is exemplified below in conjunction widi isolation of an important hematopoietic progenitor cell characterized antigen- ically as a CD34 + cell, which can be isolated from blood or bone marrow. Specifically, these rare blood progenitor cells aggregate or o ierwise increase in density, when suspended in OCS particle-based density material. Moreover, sterilization of an OCS particle-based suspension by ionizing radiation results in a composition that is toxic to these cells. Experiments demonstrating iese results and die improvements provided by the discoveries of ie present invention are presented in the sections mat follow. C. Protection by Polylactams against Radiation-induced Toxicity of Silanized Silica Particles There are a number of methods of sterilizing density gradient materials. Silica-particle based density gradient materials can be sterilized by autoclaving or by filtration, according to methods known in die art. One method d at is particularly useful is sterilization by ionizing radiation. This mediod may be preferred when d e prepared density material is to be dispensed in plastic consumable containers, such as in pre-filled centrifuge tubes, bags, or syringes and fhe like for clinical separations, or when it is to be dispensed in any heat-sensitive containers, such as plastic bottles. Unlike other methods of sterilization, irradiation-based sterilization mediods can be carried out on d e end-product container as a one-step process. That is, when it is necessary to sterilize bo a container and a liquid or suspension in the container, mere is no need to separately sterilize the container itself, as is required when filtration mediods are used, and there is no need to transfer die material from a sterilizing vat to individual containers, as would normally occur with heat-sterilization (autoclave). Techniques for sterilizing medical devices by ionizing radiation are known in the art. Commonly used me iods applicable to die present invention include gamma irradiation and electron beam (E-beam) radiation. While each of ese methods involves a different type of energy source, bodi methods involve passing die device mrough a radiation field to achieve a specific radiation dose to die device. Typically, sterilization of a medical device requires a dose of between 10 and 30 kiloGrays (kGy). However, as described below, when eiϋher of diese mediods is used to sterilize organosiianized colloidal silica, d e material becomes toxic to hematopoietic cells, unless a polylactam such as PVP is present in the particle suspension during irradiation. The polylactams used in the present invention are preferably soluble polymers such as die KOLLIDON series of polyvinylpyrrolidones (BASF, Ludwigshafen, Germany). These polymers are generally available in polymer molecular weights ranging from 2000 to 350,000, and are rated by ϋ eir intrinsic viscosities or "K values." In experiments carried out in support of the present invention, preparations having average molecular weights of about 10,000 were used. Polyvinylpyrrolidone (PVP) is known to be sensitive to gamma-irradiation. Generally, an increase in average polymer molecular weight is observed when a PVP solution is exposed to 10-50 kGy of gamma radiation. Accordingly, the manufacturer does not recommend that die material be exposed to ionizing radiation for purposes of sterilization (KOLLIDON: Polyvinylpyrrolidone for die Pharmaceutical Industry, BASF Aktiengesellschaft, Feinchemie, D67056 Ludwigshafen). In experiments carried out in support of the present invention, it has been found diat incubation of peripheral blood mononuclear ceils (PBPC) in an OCS particle-based density gradient material sterilized by filtration (0.2 μm) or by heat-sterilization (autoclave, 15 min. at 120°C) does not significantly affect recovery of viable hematopoietic progenitor cells, as assessed by a functional cell assay, die colony forming unit (CFU) assay, which measures die potential of such cells to form hematopoietic colonies. However, when an OCS particle-based density gradient material was sterilized by exposure to gamma radiation (2.5-3.5 megaRads) according to standard mediods, cells incubated in d is medium according to me protocol described as Method 2 in Example 4 (incubation with OCS + 0.5% PVP for 30 minutes plus 10 minutes centrifugation time) exhibited reduced functional activity (to approximately 25% of added CFU activity). The result was even more striking for cells treated according to die protocol described as Mediod 4 in Example 4 where no culturing step was included. Here, cells were incubated for 30 minutes in an OCS particle-based density gradient material, OCS particles + 0.5% PVP or buffer + 0.5% PVP, followed by centrifugation for 10 minutes followed by 24 hours incubation in ISCOVES medium supplemented with 10% fetal cell serum. Inclusion of 0.5% PVP in e OCS particle suspension during preparatory sterilization and resulted in a recovery of about 65% of the CFU added (FIG. 1), while those cells incubated in an OCS particle-based density gradient material alone exhibited only about 1 % of original functional activity. While is experiment used cells derived from PBMC, bone marrow- and cord blood-derived cells exhibit die same susceptibilities. The amount of polylactam required to provide protection to cells was investigated in further experiments carried out in support of die present invention. FIG. 2 shows results from OCS particle-based density gradient material was supplemented widi various concentrations of PVP prior to gamma irradiation (2.5-3.5 megaRads). In this study, PBPC cells were processed according to die protocol identified as Mediod 6 in Example 4 (incubation in presence of density gradient suspension and 10% fetal calf serum for 24 hours at room temperature). PVP in excess of 3% final concentration in d e density gradient medium provided significant protection against irradiation-induced toxicity at die dose (2.5-3.5 megaRads) of radiation applied. Addition of PVP after sterilization of the medium did not provide a similar protective effect. From the foregoing it will be apparent that die present invention includes an improved cell density gradient composition for use in separation of rare blood cells. The product composition is a silanized silica particle treated by die process of ionizing radiation in die presence of at least about 0.05% polylactam, and more specifically, between about 0.1 and 10% polyvinylpyrrolidone. Such a composition is particularly well suited for isolating rare blood progenitor cells, such as hematopoietic progenitor (CD34 + ) cells, diat are sensitive to exposure to irradiated organosiianized colloidal silica. D. Polylactam Improvement of Recovery of Rare Blood Cells from Density Gradients In accordance widi anodier aspect of ie invention, it has been discovered that supplementation of a suspension of organosiianized silica particles widi 0.05% (wt/vol) polylactam results in improved yields of rare blood cells in functional form. More specifically, inclusion of die polylactam prevents aggregation or clumping of fhe cells to provide a cell composition having a more uniform density profile and improved functional properties. In comparison to cells isolated in OCS density gradient material in the absence of a polylactam, such compositions are characterized increased recovery of rare cells in enriched fractions, and increased recovery of rare cells having clonogenic potential. FIGS. 3 and 4 show d e results of experiments in which PBPC isolated according to standard mediods were loaded onto an organosiianized colloidal silica density gradient (OCS particle-based density gradient material, 1.0605 g/ml, 280 mOsm/kg H 2 0, pH 7.4) and centri¬ fuged according to the protocol detailed in Example 3. FIG. 3 shows mat in die absence of PVP, most of the cells present in die starting material were recovered from the pellet. Visual inspection revealed diat this pelleting of die cells appeared to be die result of aggregation or clumping of the cells. In contrast, when d e OCS density material was supplemented widi 0.5% PVP, a higher percentage of cells remained at die interface, as would be predicted based on d e pre-determined density of diese cells. This observation was more pronounced when die same cell preparations were analyzed for die presence of hematopoietic progenitor cells (CD34 + cells) according to mediods detailed in Examples 5 and 6 (FACS and CFU analysis). The results shown in FIG. 4 show diat in d e absence of PVP, about 90% of the CD34 + cells migrated to the pellet. The presence of 0.5% PVP in fhe density gradient suspension reversed diis pattern. Here, most of the CD34 + cells remained at die interface, as is desirable for purposes of isolation and enrichment of such cells for transplantation and odier clinical puφoses. III. Mediods of Isolating Specific Cell Types In view of die foregoing, it can be appreciated diat die present invention includes not only a density gradient composition mat improves recovery of rare blood cells, but also a mediod of isolating functional rare blood cells, and particularly cells such as hematopoietic progenitor cells (e.g.. CD34 + cells), dendritic cells, natural killer cells, natural suppressor cells, and die like from a cell mixture. Generally, hematopoietic progenitor cells can be isolated from bone marrow, cord blood, or from peripheral blood samples derived from a subject, particularly a human subject. It is desirable to first mobilize die cells in die subject by administering agents such as granulocyte colony stimulating factor (G-CSF), VP-16, or a combination of the two factors, according to protocols well known in the art. Following such treatment, die patient is subjected to apheresis to collect PBPC or to bone marrow aspiration to collect bone marrow. According to die present mediod, die collected cells are layered onto an density gradient material, and preferably, an organosiianized colloidal silica particle-based cell separation density gradient suspension diat has a specific density diat is approximately equal to mat of die cells to be isolated. The medium preferably includes at least about 0.05% PVP, and more preferably, at least about 0.5% PVP. The suspension is then centrifuged to pellet cells having a specific density diat is greater than die specific density of fhe selected hematopoietic cell. Such a density gradient composition also has die advantage of being capable of being sterilized by mediods described above. The foregoing mediod can also be enhanced by adding to die initial cell mixture a suspension diat includes an organosiianized silica particle to which is attached a cell antigen- specific binding molecule for negative or positive selection. As an example of negative selection, die antigen-specific binding molecule binds an unwanted cell type in d e cell mixture to cause die cell to migrate to die pellet. As an example, it is sometimes desirable to deplete T-cell lymphocytes from a cell mixture, such as a hematopoietic stem cell mixture, prior to re- introducing die selected cells to die body. Such T-cell lymphocytes can be depleted, for example, by treating die cell mixture widi beads to which a T-cell lymphocyte-specific antigen is bound. The beads are d en separated from d e desired cell mixture by centrifugation. Examples of T-cell lymphocyte specific antigen binding reagents that can be bound to beads include anti-CD3, anti-CD4 and anti-CD8 mouse monoclonal antibodies. Any tubes suitable for use in centrifugation may be used for ie practice of the invention. In a preferred embodiment of d e present invention, the cell-trap centrifugation apparatus described in WO 96/07097 will be used for die density separation of CD34 + cells. A. Isolation of Hematopoietic Progenitor CD34* Cells Co-owned PCT application PCT/95/11169 publication WO 96/07097 incoφorated herein by reference, describes studies in which "PERCOLL" was used at physiologic osmolality of 280 ± 10 mOsm/kg H 2 0 and pH of 7.4 as die density gradient material to isolate CD34 + cells from apheresed blood. According to die invention described d erein, at mis osmolality, CD34* cell recovery was optimal at a specific density of 1.0605 gr/ml. In studies carried out in support of d e present invention, bone marrow cells were collected by aspiration, according to standard clinical procedures, from healdiy donors were obtained from the Bone Marrow Transplantation Laboratory of die Stanford University School of Medicine, Palo Alto, CA. CD34 + cells were enriched from the bone marrow cell fraction by centrifugation over an OCS gradient having a specific density of 1.0685 gr/ml at 280 mOsm/kg H 2 0, pH 7.4, prepared and formed in accordance with die mediods of fhe present invention, as described in Example 10. FIGS. 8 and 9 summarize results of isolation studies on bone marrow cells processed on die 1.0685 gr/ml OCS suspension. FIG. 8 shows percent recovery of total cells, CD34 + cells, T lymphcytes and red blood cells (RBC) from the entire gradient (interface + pellet). FIG. 18 shows d e percent recovery of the same cell fractions from die interface of the gradient following centrifugation. Data presented in bodi figures were calculated by dividing each fraction by fhe total number of cells initially loaded on die gradient. From FIG. 8, it can be seen mat over 90% of cells initially loaded on die gradient were recovered from die interface and pellet. Since d e loss of CD34 + cells was comparable to die approximate loss of total cells (10%), there was no preferential loss of these cells. FIG. 9 shows diat while only 26 + 10% of total cells was recovered from die interface, this fraction contained 91 + 9% of the CD34 + cells, 10% of T lymphocytes, and essentially no red blood cells. These studies demonstrate substantial enrichment of CD34 + cells, coupled widi depletion of T lymphocytes, by e density gradient procedure of die present invention. As discussed in die following section, iese characteristics recommend die preparation for use in allogeneic stem cell transplantation. B. Natural Suppressor Cells In vitro studies have shown diat human bone marrow contains low density cells diat block in vitro alloresponses in die mixed lymphocyte reactions (MLR). Based on die fact at diis suppressive activity was HLA non-restricted, it was referred to in the literature as natural sup- pressor (NS) activity. A "PERCOLL" density gradient was adjusted to a density of 1.0605 ± 0.0005 gr/ml to test for its ability to enrich cells with NS activity. Apheresed blood samples from lymphoma patients and from normal individuals d at received G-CSF treatment were cen¬ trifuged on a discontinuous five layer gradient, and die interfaces and pellet were screened for their potential to suppress die mixed lymphocyte culture. FIG. 10 shows mat cells widi NS activity had a density equal or lighter than 1.0605 gr/ml. Consequently, more man 90% of the NS activity was present in d e final cell preparation after centrifugation on a 1.0605 gr/ml gradient. C. Natural Killer Cells Natural Killer (NK) cells have been shown to kill autologous tumor cells. From a clinical perspective, it may be beneficial to have increased numbers of NK cells in die transplant to reduce tumor relapse. In experiments carried out in support of die present invention d e density of the NK cells was determined on a discontinuous five-layer "PERCOLL" gradient. NK cells also showed a density equal to or lighter dian 1.0605 gr/ml. Consequently, more man 90% of NK cells was present in die final cell preparation after centrifugation on a 1.0605 gr/ml gradient, as shown in FIG. 11. D. Enrichment of Dendritic Cells and Cytotoxic T Lymphocytes Dendritic cells (antigen presenting cells) are cells diat are derived from blood fractions. These cells act in vivo to present antigen to T lymphocytes to induce a primary cytotoxic T lymphocyte (CTL) response. These cells have clinical utility for use in inducing or enhancing cellular immune responses. According to anod er aspect of the present invention, dendritic cells and cytotoxic lymphocytes are isolated using defined density gradients, in a diree step process. This process includes centrifugation dirough diree different density gradient solutions, as detailed in Example 12. In accordance widi die present invention OCS density gradient material having densities of 1.0770, 1.0720 and 1.0610 or 1.0720, 1.0610, and 1.0565 gr/ml, each at pH 7.4 and an osmolality of 280 mOsm/kg H 2 0 are used in die diree steps, which are conveniently, but not necessarily carried out in a cell-trap tube. Practitioners will recognize diat equivalent densities of other media can be used as well. For example, step one can be carried out in Lymphoprep (Nycomed Laboratories, Oslo, Norway) or inFicoll equivalent PERCOLL (density 1.0770, 310 mOsm/kg H 2 0, pH 7.4); step two can be carried out in a PERCOLL solution (density 1.0650, pH 7.4, 300 mOsm/kg H 2 0; and step diree can be carried out in a PERCOLL solution having a density of 1.0800 gr/ml, pH 7.4, 540 mOsm/kg H 2 0, or in a PERCOLL solution having a density of 1.0550 gr/ml, 290 mOsm/kg H 2 0, pH 7.4). However, die foregoing densities determined using OCS-particle based cell separation and die enrichment mediods of die present invention, are advantageous in diat d ey are carried out at physiological osmolalities, rather dian under the hyperosmotic conditions previously reported. E. Density Adjusted Cell Sorting (OACS) The present invention also provides for purging from die bone marrow cells or other progenitor cell source contaminating tumor cells or other undesirable cells. For puφoses of purging, d e density of a given tumor cell is determined by centrifuging a tumor containing sample on a discontinuous density gradient ranging from 1.0490 to 1.0640 gr/ml. The majority of undesirable, non-tumor cells represent cells of the immune and hematopoietic system and have a density in die range of 1.0610 to 1.0770. The density of tumor cells generally falls widiin the 1.0490 to 1.0580 gr/ml density range. The density of die cell separation medium is determined to an accuracy of within ± 0.0005 gr/ml, preferably ± 0.0002 gr/ml of the specific gravity of the desired cells. Thus tumor purging can be accomplished by a two step process, in which die desired cells are first pelleted dirough a cell separation medium capable of retaining tumor cells. Alternatively, the purging technique can be used to remove tumor cells in a one-step process, according to the mediods described below. Experiments carried out in support of d e present invention demonstrated mat CD34 + cells were enriched using die DACS procedure in conjunction widi die density method of d e inven- tion, by removing contaminating cells wi i an anti-CD45 mAb coupled to a heavy carrier (such as magnetic beads or aminopropyl glass beads). The total cell number was reduced 82% and d e CD34 yield was around 40% . CD34 purity was increased from 2% to approximately 20%. Since the anti-CD45 antibody removed also some ofthe CD34 + cells, it is appreciated that ϋ is mediod could be improved by using a mixture of odier antibodies to deplete non-stem cells. To examine non-specific cell losses during DACS, experiments were performed using buffy coat peripheral blood mononuclear cells (PBM) and DACS beads in die absence of monoclonal antibodies. As detailed in Example 10, DACS beads were made using eidier intact GAM-IgG or F(ab) 2 GAM-IgG to test die dieory that Fc receptor mediated binding of cells to IgG-coated DACS beads contributed to non-specific depletion. The results in FIG. 5 de on- strate mat there was no non-specific depletion of cells when GAM-IgG F(ab) 2 coated DACS beads were used compared to DACS beads widi intact GAM-IgG. DACS beads coated widi GAM-IgG, linked via die glycosylated site on die Fc region, also demonstrated reduced non-specific depletion of cells when compared widi DACS beads widi GAM-IgG in a random orientation. These results demonstrate diat removing or blocking the Fc region of die GAM-IgG leads to a reduction in non-specific depletion of cells. The foregoing results show diat Fc Receptor (FcR)-mediated binding of cells to DACS beads may be responsible for some of die non-specific depletion of CD34 + cells observed during DACS treatment. This is further illustrated by die site-specific coupling which results in a reduction of non-specific loss of cells due to blocking of die glycosylated areas of the Fc portion of die immunoglobulin molecule, as described above. However d e use of BSA-coated DACS beads has demonstrated d at FcR-mediated binding is not usually responsible for all the non-specific depletion of cells. Ionic interactions between die cell and the bead surface are also likely to be involved. To investigate diis possibility, DACS beads coated widi PEG sandwiched between d e bead surface and die GAM-IgG were made. This was done to exploit die ability of PEG to reduce ionic interactions between proteins and charged surfaces. In an experiment comparing PEG-GAM-IgG-coated DACS beads with DACS beads wi i GAM-IgG in the absence of PEG, it was discovered that die use of PEG is associated wid a reduction in non¬ specific depletion of cells. Reduction in non-specific depletion by modification of die DACS bead coating is particularly important in the context of the negative enrichment of rare cells such as CD34 + cells, NK cells, natural suppressor cells, dendritic cells, fetal nucleated cells, and die like, where it is important to maximize the yield of cells. For example, experiments to define d e non-specific loss of CD34 + cells demonstrated diat die use of F(ab) 2 coated DACS beads lead to a reduction in die non-specific loss of CD34 + cells (FIG. 6). Likewise, using DACS beads coated widi GAM-IgGl bound via die glycosylation site of the Fc region also reduced non-specific depletion of CD34 + cells when compared widi DACS beads widi GAM-IgG in a random orientation (FIG. 7). E. Depletion of T-cell Lymphocytes Graft versus host disease (GvHD) is induced by T lymphocytes mat are present in donor allografts, though complete removal of such cells also results in failure of die graft and tumor relapse. Thus die presence of a limited number of T lymphocytes may be required for successful allogeneic transplant. In experiments carried out in support of d e present invention, DACS beads were used to deplete T lymphocytes from apheresed PBSC preparations from G-CSF mobilized donors. T lymphocytes were depleted from a mobilized PBSC preparation by adding to die preparation of mixture of anti-CD3, anti-CD4 and anti-CD8 mouse monoclonal antibodies, followed by goat anti-mouse-IgG DACS beads, and centrifugation dirough an OCS gradient having a density of 1.0605 gr/ml at 280 mOs , pH 7.4. T lympho- cytes were monitored by reaction widi anti-CD2 monoclonal antibodies, in order to avoid possible artifacts associated wid using antibodies that recognize he same epitopes on the differentiation antigens. The foregoing DACS T lymphocyte depletion method resulted in a 97% reduction in T lymphocytes widi only a 70% reduction in die yield of CD34 + cells. Sixty percent of the CD34 + cells were retained in the interface after DACS treatment. The results demonstrate mat d e combined cell enrichment and DACS method specifically reduces T lymphocyte content of allograft material by 97% widi less dian 40% loss of CD34 + cells. Such a preparation is suitable for preventing GVHD in transplant material. F. Enrichment and Purging of Tumor Cells 1. Enrichment of Tumor Cells for Diagnosis Breast tumor cells are used to exemplify die enrichment mediods of the present invention. To enrich for such cells, a gradient should be adjusted to a density of 1.0490 to 1.0580 ± 0.0005 gr/ml, depending upon die specific type of breast tumor cell, a physiologic osmolality of 270-290 mOsm/kg H 2 0 and physiologic pH 6.8-7.8. In a specific embodiment by way of examples, breast tumor cells are directly loaded into a cell-trap centrifugation tube containing an appropriate cell separation medium, such as OCS-particle-based cell separation medium or "PERCOLL" solution filled to a level above the constriction. In d e specific examples described herein, successful enrichment of rumor cells was carried out on "PERCOLL" density gradient material which had been adjusted to the specific density of between 1.0490 to 1.0580 ± 0.0002 gr/ml, depending upon the specific type of breast tumor cell, osmolality of 280 mOsm/kg H 2 0 and pH 7.4. The density of die "PERCOLL" solution may be adjusted on a densitometer to precisely define its accuracy. The cells enriched by die methods described above may subsequently be examined for the presence of breast tumor cells. The resultant yield of isolated or enriched cells from die cell separation mediods of d e present invention may be used for diagnostic puφoses, e.g. moφhological, molecular, biochemical or immunophenotypic assays. For example, DNA may be prepared from the collected cells and subjected to polymerase chain reaction (PCR), as described under the tumor cell purging section below, or the collected cells may be assessed moφhologically thereby avoiding die use of invasive and expensive surgical procedures heretofore relied upon for such a determination. Various breast tumor antigens and breast tumor markers are known to d ose of skill in the art or are commercially available including but not limited to cadiepsin D, EGF-R , estrogen receptor, Ki-67, progesterone receptor and TGF-α. Antibodies directed to diese antigens or markers may be used to assess die tumor type of collected cells. A major advantage of die mediods described herein is that a large volume of complete blood may be directly placed on the density gradient. Peripheral blood may be collected in anti-coagulant-containing tubes or by apheresis or leukopheresis. Complete blood does not need to be processed or diluted prior to centrifugation. However, since die mediods enrich breast tumor cells based on their specific buoyant density, it is important d at die cells are subject to separation within a relatively short time after their collection from an in vivo source because the density of the cells changes according to dieir culture or storage conditions. Therefore, in order to obtain optimal enrichment of breast tumor cells from blood, it is preferred that the blood samples are used within 48 hours after their collection. Most preferably, blood samples should be subjected to density gradient centrifugation within several hours of collection. 2. Purging of Tumor Cells Tumor cell purging is conveniently carried out using die tumor cell enrichment mediods discussed above, or more preferably, the DACS augmented cell separation mediods described above. By way of example, when radioactively labeled breast tumor cells were mixed wid a peripheral blood buffy coat, up to 80% of were retained by centrifuging the mixture on a gradient having a specific density of 1.0580 gr/ml and an osmolality of 280 mOsm/kg H 2 0. In addition, only a small fraction (< 10% of initial cell number) of non-tumor cells were found in die collected tumor fraction. Co-owned PCT application PCT/95/ 11169 publication WO 96/07097 describes enrichment of breast tumor cells according to the mediods of the invention. This method may also be used in conjunction widi d e DACS procedure described herein. For example, complete blood can be directly incubated wid carrier particle- coated-anti-CD45 antibodies which react widi most leukocytes. Since breast tumor cells do not react widi anti-CD45 to any significant degree, the vast majority of the non-red blood cells, leukocytes, and odier cells are rendered heavier d an die density material and pellet during centrifugation, while the breast tumor cells remain in die upper compartment. A variety of other cell type-specific binding agents may be used to target specific cell types in die blood, as discussed in d e previous sections. Alternatively, according to a preferred purging procedure, d e positive selection DACS procedure may be used to cause tumor cells to be heavier than dieir normal densities, so that ey are pelleted during centrifugation. Cell type-specific binding agents useful in d e positive selection procedure include, but are not limited to antibodies to breast tumor antigens and antibodies to breast tumor markers, e.g. HER2/Neu, CA 15-3, CA 549, cathepsin D, EGF-R, estrogen receptor, Ki-67, progesterone receptor, and TGF-α. Many of diese antibodies are commercially available. The cells enriched by die mediods described herein can subsequently be examined for the presence of breast tumor cells. The resultant yield of isolated or enriched cells from die cell separation mediods of the present invention may be used for diagnostic puφoses, e.g. moφhological, molecular, biochemical or immunophenotypic assays. For example, DNA may be prepared from the collected cells and subjected to polymerase chain reaction (PCR) or die collected cells may be assessed moφhologically diereby avoiding die use of invasive and expensive surgical procedures heretofore relied upon for such a determination. Alternatively, tumor cells can be identified by die presence of tumor cell markers known to those of skill in the art, as described above. Antibodies directed to diese antigens or markers may be used to assess die tumor type of collected cells. 3. Tumor Purging of Peripheral Blood Samples The presence of tumor cells in stem cell grafts and the inability to specifically deplete tumor cells from die bone marrow of non-Hodgkins lymphoma patients may be an important indicator in predicting disease relapse after autologous transplantation. Cell separation mediods in accordance widi die present invention can be applied to remove tumor cells from peripheral blood products prior to re-introduction of such products into die patient. In studies carried out in support of die present invention, die human B-cell lymphoma cell line SU-DHL4 was used to model tumor cell contamination of peripheral blood. This cell line contains a reciprocal translocation between the long arms of human chromosomes 14 and 18, resulting in a juxtaposition of die bcl-2 proto-oncogene to die joining region of the immuno¬ globulin (Ig) heavy chain gene. This translocation, which leads to elevated levels of die bcl-2 gene product, is found in 85% of follicular small cleaved cell lymphomas and 25% of diffuse large cell lymphomas. The translocation breakpoint in the bcl-2 gene is located in eid er die major breakpoint region (MBR) spanning approximately 150 bp in die 3' untranslated region (UTR) of the bcl-2 gene or die 500 bp minor cluster region (MCR) located approximately 20 kb downstream of the bcl-2 gene. Using translocation-specific primer pairs and quantitative PCR by limiting dilution analysis techniques known in die art, depletion of SU-DHL4 cells in DACS purging experiments was monitored. Likewise, human SK-BR3 adenocarcinoma cell line is used to model breast cancer cell infiltration of peripheral blood. This cell line reacts widi die mouse anti-human Her2/Neu monoclonal antibody. Prior to being spiked into normal buffy coats, die SK-BR3 cells were pre-loaded widi calcein AM, a fluorogenic substrate so diat diey could be enumerated by FACS analysis. Details of die experimental mediods used in carrying out tumor purging experiments are provided in die Examples. A PCR assay was used to monitor tumor cell levels in the blood cell mixtures. Initially, mixtures of genomic DNA from SU-DHL4 and a (14; 18) translocation-negative human cell line (K562) were used as starting templates to assess the level of sensitivity of the PCR assay. Agarose gel electrophoresis was used to separate PCR products from eidier one round or from two-rounds of amplification, as shown in. PCR products specifically derived from SU-DHL4 DNA were identified on die gel. The results of diese preliminary tests showed mat a nested PCR assay can detect approximately 1 SU-DHL4 cell per 150,000 cells. In addition, genomic DNA was prepared from frozen aliquots of apheresed blood of 24 NHL patients who received chemodierapy and G-CSF, according to standard methods known in die art. Nested PCR products using primer-pairs specific for the major breakpoint region (MBR) translocation were found in 4 patients, demonstrating that diis assay can detect cells carrying die translocation from audientic clinical samples. Further experiments were carried out to validate die use of PCR analysis under conditions of limiting dilution for quantitation of tumor cell frequency in a sample. Replicate PCR reactions were performed using known amounts of input SU-DHL4 genomic DNA. Nested PCR primer-pairs for t (14; 18) (a target sequence present once per SU-DHL4 cell) and p53 (a target sequence present twice per SU-DHL4 cell) were used. The observed frequency of positive PCR reactions demonstrated that experimental frequencies are similar to frequencies expected based on Poisson statistics. Thus, PCR using limiting dilution analysis was demonstrated to provide an accurate, quantitative me od for monitoring tumor cells. To model tumor cell contamination of PBPC, samples of apheresed blood from G-CSF mobilized patients were seeded widi a known number of SU-DHL4 cells. This spiked material was used to assess tumor cell removal by DACS. This mediod involves incubation of die blood widi mouse monoclonal antibodies specific to surface antigens expressed on die target cell (CD9, CD10, CD19, CD20). This step was dien followed by incubation with goat anti-mouse Ig coated high-density microspheres (DACS beads). The sub-population of cells bound to diese microparticles was then separated from unbound cells by centrifugation through an OCS suspension supplemented with 0.5% PVP at a specific density of 1.0605 gr/ml and sterilized by gamma irradiation. Cells were recovered from die interface (IF) and from the pellet (PT) after gradient centrifugation and were quantitated by gel electrophoresis, as detailed in Example 5. The results of a representative purging experiment are shown in FIG. 12 and FIG. 13. FIG. 12 shows die number of spiked SU-DHL4 cells in d e; A) unfractionated sample (determined by visual count), B) unfractionated sample, C) interface widiout DACS, D) interface after using GAM-IgG DACS beads, and E) interface after using GAM-IgG DACS beads and a cocktail of anti-B cell mAbs. The number of SU-DHL4 cells in B, C, D and E was determined by quantitative-PCR. FIG. 13 represents die recovery of CD34 + cells in the interface (IF) and pellet (PT) of C, D and E. Tumor cell depletion was quantitated by PCR limiting dilution analysis for samples B-E, as illustrated. The recovery of CD34 + hematopoietic stem cells was monitored by diree-color flow cytometry. Recovery of total cells and stem cells are shown relative to die recovery of cells from die interface. A single purging cycle using an antibody cocktail resulted in specific 2-3 log depletion of SU-DHL4 cells. The recovery of CD34 + cells after DACS was greater than 70% and comparable to die number of CD34 + cells recovered from the interface in the absence of DACS. Peripheral blood mononuclear cells are reduced by 70-80% by die density centrifugation with or without DACS. In ano er model for tumor cell contamination, SK-BR3 cells were seeded into normal buffy coats at a frequency of 2%. The calcein AM labeled cells tumor cells were removed from the ceil suspension by centrifugation on an OCS density suspension having a specific density of 1.0605 gr/ml with DACS using anti-human Her-2/Neu Mabs. The depletion vas determined by FACS analysis. The calcein AM labeled cells were gated and dieir percentage determined using die LYSYS II program. Whereas the SK-BR3 cells were not depleted from die buffy coat after processing on OCS suspension in the presence or absence of DACS beads, diey were almost completely removed from die cell mixture when using DACS beads in combi¬ nation with e breast cancer specific anti-Her2/Neu mAb. These data show at DACS treatment reduces tumor cell contamination by 1 to 2 logs (10-100-fold) when the FACS percentage is back calculated to die initial tumor cell number spiked in die sample. In summary, die foregoing exemplary model experiments using a human B-cell lymphoma cells and quantitative PCR demonstrate at a 2-3 log reduction in tumor cells can be achieved in one round of DACS while retaining die majority of CD34 + hematopoietic stem cells. Using a human adenocarcinoma cell line SK-BR3 and FACS analysis, a 1-2 log reduction in tumor cells was achieved. More generally, it is die discovery of die present invention at diis method can be used to purge blood or dierapeutic enriched cell populations of unwanted tumor cells. IV. Sterilized Cell Separation Kit An important application of the discovery of the present invention is a sterile kit for cell separation. Such a kit is particularly useful for clinical cell separations. Such a kit will commonly include a centrifugable container and a density gradient material for separating cells, which has been sterilized as a filled unit by exposure to ionizing radiation. The centrifugable container will commonly be a consumable, disposable unit formed from plastic, such as polypropylene, polycarbonate, polysulfone, polymediylpentene, polye ylene, polyallomer, polystyrene and die like. However, as discussed below, die advantages of sterilizing die kit as a pre-filled unit also makes diis procedure desirable for kits diat include tubes formed from other materials. In a preferred embodiment, die container will be closed, as illustrated below, to facilitate aseptic collection of starting material and preservation and manipulation of die material in a sterile condition. FIG. 14 illustrates a kit 10 formed in accordance with the present invention. As illustrated, kit 10 includes centrifuge tube 12 shown here filled with density gradient material 14. The density gradient material is formed by a suspension of organosiianized silica particles, such as particles 16. Also included in die density gradient material is at least 0.05% PVP, and preferably between about 0.1% and 10% PVP to reduce toxicity to rare blood cells isolated in die kit, dius improving yield of such cells in the kit. The container and d e density gradient contained witiiin are sterilized by ionizing radiation, such as gamma irradiation or E-beam. The resulting kit may also include a protective cover, such as cap 18 to protect die contents of the tube from contamination after sterilization. The kit may also include a stand, such as tube-holder 20, to prevent displacement of tube contents during shipment and storage. Kits as described above can be prepared according to die specifications required for separation of a number of cell types. The size of tube used, as well as its structural and chemical resiliency can be varied widiin the confines of the present invention for use in ultracentrifugation as well as in relatively low-speed centrifugation applications as described herein. Such modifications will be apparent to persons skilled in die art. Similarly, die kit may be designed for different cell types by varying die composition of d e density gradient material. Example 9 describes a kit at is particularly formulated for isolation of CD34 + hematopoietic progenitor cells from PBPC. In the embodiment described, die OCS particulate density gradient suspension is adjusted to a specific density of 1.0605 g/ml and an osmolality of 280 mOsm/kg H 2 0. This density is approximately equal to the density of the desired CD34 + cells, so that upon centrifugation, diese cells will "float" at die interface between die loading material and the density material present in the tube. It is appreciated that enrichment of other rare blood cell types, such as dendritic cells, cytotoxic T cells, natural killer cells, natural suppressor cells and fetal nucleated cells can be achieved by adjusting d e specific density and osmolality of d e density gradient material. Such adjustment can be made so at die desired cells will d en eidier float or pellet, according to die density of the desired cells and die relative density of unwanted cells present in die cell mixture from which the cells are to be isolated. A preferred centrifuge tube configuration for use in die kit and mediod of d e invention is one diat has a constriction member disposed widiin die tube, forming a "cell-trap. " Such a tube is described in co-owned PCT application PCT/95/11169 (publication number WO 96/07097) which is incoφorated herein by reference in its entirety. A modified form of this tube is illustrated as part of a closed system in FIG. 14 herein, described below. In die cell-trap tube, die constriction member is positioned and constructed to retain fluid in the bottom portion of the tube below die constriction member when the tube is inverted. The tube also includes cell-separation medium mat is contained in die bottom portion of the tube and which extends above the constriction member to a level above the opening formed by die constriction member. Cells that are captured at an interface between the cell-separation medium and a lower-density medium, after centrifugation, are discharged widi the lower- density medium when the tube is inverted. This facile means of collection of cells is particularly convenient in the context of the present invention, where minimal manipulation of the cells is desirable. It is also appreciated diat, for clinical applications, the kit can be configured as a "closed system" to restrict access of external pamogens to die biological materials, as described herein. FIG. 14 shows a centrifugeable container having a constriction member incoφorated into a closed system. As illustrated, closed system 70 includes a reservoir 71, shown as sterile bag, containing blood 72 previously collected by known techniques, is connected by sterile connecting tubing 74 to centrifuge container 76, illustrated as a style centrifuge tube having a closed top 77. The closed top will have at least one, and preferably at least two entry ports, useful for introduction and removal of sample, and for venting, as described below. In die embodiment shown, solid ridge 79 protruding upward from closed top 77 is included to form a protective barrier for the entry ports, and as an attachment point for a protective, removable lid for die apparatus diat serves to reduce potential contamination during shipping and storage. Widi further reference to FIG. 14, tubing 74 is attached to container 76 dirough entry port 78, adapted widi fitting 80, which may be any type of locking tip adapted for sterile connection, for example, a Luer-Lock™ syringe connector. Alternatively, fitting 80 may be a sterile septum adapted for connection with sterile fluid bags and tubes, for example a SAFSITE™ small wire extension set wi reflux valve and Spin-Lock™ adaptor available from Burron Medical Inc., Bethlehem, Pennsylvania. To facilitate fluid flow into centrifuge tube 76, the bucket contains air vent entry port 82. As shown, air filter 84 is attached to entry port 82 to prevent contamination. In accordance widi a preferred embodiment of d e invention, container 76 includes constriction member 88. Although a number of configurations are possible, as illustrated, constriction member 88 is funnel shaped on its upper surface. As illustrated, constriction member 88 is formed integrally with container 76 forming an indentation 89 on the outer surface of the tube. The constriction member is supported by supports 90 to prevent compression during centrifugation. The container also contains OCS particle density gradient material 91 disposed in die tube bodi above and below constriction member 88. The entire system as illustrated is sterilized by gamma irradiation or E-beam according to standard mediods. An added feature of container 76 is entry port 92. This entry port communicates via closed fluid channel 94 widi the bottom portion 95 of container 76. The entry port and channel are used to fill the lower portion of the tube 95, for example, with density gradient cell separation medium prior to sterilization. Alternatively, the port and channel may be used to remove materials, including cell pellet materials, from the bottom of the tube following centrifugation. This feature of the tube is neidier required by nor restricted to the closed system context in which it is illustrated. Fluid flow from reservoir 70 into container 76 can be initiated by applying suction on air vent entry port 82, or it may be initiated by otiier means known in the art, including gravity. The rate of flow is adjusted, ei ier by altering die pressure head between die two containers or by regulating a valve positioned in the tube or at die entry port at a point between reservoir 70 and entry port 78. Flow rate is optimally regulated to fill or partially fill the upper portion of container 76, above die level of the density gradient solution. When sufficient fluid sample has entered die tube, flow can be terminated by any of a number of means known in the art, such as regulation by a valve or by lowering of pressure head. The tubing is then removed from die bucket, port 78 is sealed, and the closed container is subjected to centrifugation as described above. In anotiier preferred embodiment that is particularly useful in carrying out the mediod of the present invention, die constricted tube may take the form of a centrifugeable syringe, such as the centrifuge syringe described in co-owned PCT application PCT/US95/11162 (WO 96/06679) which is incoφorated herein by reference. This embodiment is well suited for use in a closed system as described above. Anotiier configuration of the device tiiat is suitable for use in the invention is a centrifugeable bag. Such a configuration is particularly suitable for use in processing clinical blood samples which can be collected directly into the bag. The following examples illustrate, but are in no way intended to limit the present invention. EXAMPLES Materials A. Density Gradient Materials Organosiianized colloidal silica particles (size range: 15-30 nm) used in d e examples below were prepared according to the methods described in U.S. Patent 4,927,749 using gammaglycidoxypropyl-trimethoxysilane (GPMS). Polyvinylpyrrolidone (PVP-10) was obtained from Sigma (St. Louis, MO). B. Monoclonal Antibodies Monoclonal antibodies (mAb) directed against surface antigens specific for hematopoietic progenitor cells (anti-CD34; anti-HPCA-2) and leukocytes (anti-CD45; anti-HLE-1) were ob¬ tained from Becton Dickinson, Inc. (San Jose, CA). The different monoclonal antibodies were directly labeled witii fluorescein isotiiiocyanate (FITC) or phycoerydirin (PE). PE labeled isotype control IgGl polyclonal antibodies were obtained from Becton Dickinson, Inc. (San Jose). Example 1 Preparation of Density Gradient Solution The OCS particle-based density gradient material prepared as described under "Materials" above was diluted in phosphate buffered saline (PBS) to provide a suspension having a defined osmolality and specific density at pH 7.4, as indicated. The OCS particle-based density gradient material was diluted in water to die approximate specific density required. Solid salts were added to provide a final physiological salt solution (physiological saline or Dulbecco's phosphate buffered saline; Gibco). The suspension was supplemented with different final concentrations (0 % - 10 % w/w) of polyvinylpyrrolidone (PVP- 10) by adding die solid powder adjusted for d e density of die final suspension. The diluted suspension was finally sterilized by ionizing irradiation (gamma irradiation and E-beam, 2.5-3.5 megaRads; Isomedix, Morton Grove, IL), autoclaving (15 min. at 120°C) or filtration dirough a 0.2 micron filter according to standard mediods known in die art and used at room temperature. Example 2 Preparation of Hematopoietic Progenitor Cells A. Blood Cell Mixtures Peripheral blood mononuclear cells (PBPC) were collected by apheresis from non- Hodgkins lymphoma (NHL), Hodgkins lymphoma (HL) and breast-cancer patients at the Bone Marrow Transplantation Laboratory at the Stanford University School of Medicine, Palo Alto, California, USA. PBPC were mobilized in NHL patients by treatment widi cyclophosphamide (4 g/m 2 , intravenously) followed by granulocyte colony stimulating factor (G-CSF) (10 μg/kg, intravenously, daily). PBPC were mobilized in breast-cancer patients by treatment with etoposide (VP-16; 2g/m 2 , intravenously) followed by G-CSF (10 μg/kg, intravenously, daily). PBPC were mobilized in HL patients by treatment widi G-CSF alone. The mobilization protocols used are standard protocols known in die art. Twenty-four hours following the final injection, each patient was subjected to apheresis. PBPC were collected from the apheresed blood according to standard mediods. B. Bone Marrow Cell Mixtures Bone marrow aspirate cells were layered slowly on an OCS suspension prepared as described above to a density of 1.0685 gr/ml at an osmolality of 280 mOsm/kg H 2 0. A maximum of 2xl0 9 cells was layered and processed per centrifuge tube. The tube was centrifuged for 30 minutes at 850 x g, at room temperature. Cells from the interface and pellet were collected separately from a cell-trap tube, in accordance witii die present invention. Cell fractions were characterized by FACS analysis using FITC- and PE- conjugated anti-CD34 (anti-HPCA-2), anti-CD45 (anti-HLe-1), anti-CD3 (Leu-4) and IgGl antibodies, obtained from Becton Dickinson, Inc. (San Jose, CA). For analysis, cells were labeled widi the nuclear dye LDS 751 (Exciton, Inc., Dayton, OH). Statistical analysis was performed on 10* flow events using a FACSCAN system equipped widi a LYSYS II program (Becton Dickinson, Inc. (San Jose, CA). Example 3 Density Gradient Separation of PBPC PBPC cells (025ml), collected according to Example 2 and diluted PBS at a concentration of approximately 5 x IO 7 cells/ml) were layered on an OCS particle-based density gradient suspension (15 ml in a 50 ml centrifuge tube). The suspension had a specific density of 1.0605 gr/ml, an osmolality of 280 mOsm/kg H 2 0 and a pH of 7.4. The layering was performed slowly to avoid mixing of the sample widi the solution. A maximum of 2 x 10' cells was layered per tube. The centrifugation was performed for 30 min. at 850 x g at room temperature. To prevent mixing of the cells and the density gradient solution, the centrifuge was stopped widiout braking force. Following centrifugation, die cells were divided into a low-density fraction located at die interface and a high-density fraction forming die pellet. Each cell fraction was collected and transferred into anotiier 50 ml polypropylene centrifuge tube. The cells were washed once and stored in Ca + + and Mg + + free Dulbecco's phosphate buffered saline (D-PBS) at room temperature until further manipulation. The number and functionality of the hematopoietic progenitor cells (CD34 cells) was determined in botii cell fractions by FACS analysis and clonogenic assays (CFU) as detailed in Examples 5 and 6, below. Example 4 Effect on Cells of Mixing with Silanized Colloidal Silica Particles Assessment of the effect of mixing and subsequent incubation of human peripheral blood in silanized colloidal silica particle-based density gradient solutions was carried out by one or more of the mediods described below. After d e respective incubations as detailed below, d e cells were collected and washed once wid PBS. Their ability to form hematopoietic colonies (CFU-E, BFU-E, CFU-GM and CFU-GEMM), according to mediods known in the art, and as described in Example 6, below, was screened at die end of each mediod. In carrying out experiments, Methods 1, 3, and 5 are control incubation conditions for Mediods 2, 4, and 6, respectively. Metiiod 1: An aliquot containing 10M0 8 cells in 5 ml of 1 X D-PBS (Ca + + , Mg + + free) was added to a 15 ml polypropylene centrifuge tube. Initial cell number was noted. The cells were incubated for 30 minutes at room temperature widi gentle mixing at 15 minute intervals. Metiiod 2: An aliquot containing 1OM0 8 cells in 5 ml of OCS density gradient suspen¬ sion was added to a 15 ml polypropylene centrifuge tube. Initial cell number was noted. The cells were incubated for 30 minutes at room temperature with gentle mixing at 15 minute intervals. Method 3: An aliquot containing 10 7 -10 8 cells in 5 ml 1 X D-PBS (Ca + + , Mg* + free) was placed in a 15 ml polypropylene centrifuge tube. Initial cell number was noted. The cells were incubated for 30 minutes at room temperature, with gentle mixing at 15 minute intervals. The ceil mixture was then centrifuged at 550 x g for 10 minutes. Following centrifugation, die supernatant was discarded and the cell pellet was re-suspended in 5 ml of Iscove's medium supplemented widi 10% fetal calf serum. The cell mixture was then incubated for 24 hours at room temperature. Mediod 4: An aliquot containing 10M0 8 cells in 5 ml OCS density gradient suspension was placed in a 15 ml polypropylene centrifuge tube. Initial cell number was noted. The cells were incubated for 30 minutes at room temperature, widi gentle mixing at 15 minute intervals. The cell mixture was then centrifuged at 550 x g for 10 minutes. Following centrifugation, the supernatant was discarded and die cell pellet was re-suspended in 5 ml of Iscove's medium supplemented with 10% fetal calf serum. The cell mixture was then incubated for 24 hours at room temperature. Method 5: An aliquot containing 10 7 -10 8 cells in 5 ml of 1 X D-PBS (Ca + + , Mg + ' free) supplemented with 10% fetal calf serum was added to a 15 ml polypropylene centrifuge tube. Initial cell number was noted. The cell mixture was then incubated for 24 hours at room temperature. Method 6: An aliquot containing IO 7 to IO 8 cells in 5 ml of OCS density gradient suspension supplemented widi 10% fetal calf serum was added to a 15 ml polypropylene centrifuge tube. Initial cell number was noted. The cell mixture was men incubated for 24 hours at room temperature. Example 5 Staining and Quantification of CD34+ Cells by FACS CD34 + cell quantity was determined by Fluorescence Activated Cell Sorting (FACS) after die cells were labeled widi a nuclear dye and mAbs directed to CD34 and CD45. The percentage of CD34 cells present was determined in die gate of nucleated cells. This approach was chosen to avoid interference of non-nucleated particulate material with the accuracy of CD34 cell analysis in the FACS. A suspension of 2xl0 7 cells/ml was made, using Ca + + and Mg + + free D-PBS as diluent. Fifty microliters of this cell suspension containing lxlO 6 cells was added to each well of a 96- well microtiter plate. Fifty microliters of a 20% heat inactivated rabbit serum/D-PBS solution and 10 microliters of 10 μg/ml LDS 751 (nuclear staining dye) in D-PBS solution were also added to each well, followed by mixing. The plate was covered wid foil and incubated for 30 minutes at room temperature. To each control well, was added 10 microliters IgGl-PE. To each test well, was added 10 microliters Anti-CD34-PE, followed by mixing. The plate was covered with foil and incubated for 15 minutes at 4°C. The plate was dien centrifuged at 850 x g for 5 minutes at 4"C. The supernatant was then removed by flicking. Each cell pellet was re-suspended in 200 μl of cold (4°C) lxDPBS (Ca + + and Mg + + free). The plate was dien centrifuged at 850 x g for 5 minutes at 4°C, and the resulting supernatant was removed by rapid flicking of me plate. Each cell pellet was re-suspended in 50 μl 20% heat inactivated rabbit serum solution. anti-CD45-FITC (10 μl) was added to each control and test well and mixed. The plate was covered widi foil and incubated for 30 minutes at 4'C. One hundred microliters of cold (4'C) lxD-PBS (Ca + + and Mg + + free) was then added to all control and test wells, and die plate was centrifuged at 850 x g for 5 minutes at 4 * C. The plate was flicked rapidly to remove the supernatants, and each cell pellet was re-suspended in 200 μl cold (4 * C) lxD-PBS (Ca + + and Mg + + free). The plate was centrifuged at 850 x g for 5 minutes at 4 * C, then flicked rapidly to remove supernatant. Each cell pellet was then re- suspended in 200ml 1 % paraformaldehyde (4°C). The plate was dien covered widi foil and stored at 4 * C until FACS analysis of die samples. FACS analysis was performed on IO 4 flow events using a FACSStar Plus system equipped widi a LYSYS 11 statistical analysis program (Becton Dickinson, Inc.). For puφoses of analysis, a gate (Region 1) was placed around die nucleated cells determined by LDS 751 staining in FL3 in order to gate out die red cells, platelets and debris. FLl and FL2 were displayed as a dot plot using Region 1. A gate (Region 2) was placed around die cell population diat stained witii both the anti-CD45 and anti-CD34 monoclonal antibodies. For puφoses of analysis, the percentage of cells that stain witii the anti-CD34 (FL2) and anti-CD45 (FLl) monoclonal antibodies was determined in Region 2. This represents die number of CD34 positive cells as a percentage of the total number of nucleated cells. The total number of CD34 positive cells was determined in the unprocessed sample and in the cell fraction obtained from die interface and the pellet after processing of the PBPC samples in the density gradient solution. Example 6 Colony Forming (CFU) Assay The functional characteristics of the CD34 + cells in a cell sample were determined by die colony formation (CFU) assay. This assay provides quantitation of the number of committed hematopoietic progenitor cells in die cell solution. Cells were diluted to a concentration of IO 5 cells in 2mL commercially prepared semi- solid mediyl cellulose containing various colony stimulating factors and erythropoietin (MethoCult™ H4433 medium, Terry Fox Laboratories, Vancouver). After 14 days of culture at 37°C, die eryd roid (CFU-E, BFU-E), granulocyte/macrophage (CFU-GM) and mixed (CFU-GEMM) colonies were counted under an inverted microscope (40x), according to standard methods. The total number of CFU present in a cell mixture was defined by summation of the different types of colonies counted. Example 7 Determination of Total Cell Recovery The percent cell recovery was determined by die formula: % Recovery = 100 x r. ∞ ot S S^S? ™ The percent CD34 cell recovery was determined by d e formula: < & P p fv i p r - inn Y Nr. of CD34 + cells after manipulation % Recovery - 100 x Number of CD34 cells at start The percent clonogenic cell recovery was determined by die formula: % u < . mυ p « - ^nn Y Nr. clonogenic cells (CFU) after manipulation % Recovery - 1U0 x Nr. ot clonogenic cells (CFU ) a t start Example 8 Effect of Radiation on hematopoietic cell recovery The effect of irradiated organosiianized colloidal silica particles on hematopoietic cell integrity was tested, using die protocol described in Example 4 as "Methods 3 and 4." OCS particles, OCS particles supplemented wid 0.5% PVP, or PBS supplemented widi 0.5% PVP (5 ml total volume) was irradiated in a 50 ml conical centrifuge tube by a dose of gamma irradiation (2.5-3.5 megaRads). In diese experiments, die density of the OCS particle suspension used was 1.0605 gr/ml, a density diat is lower tiian diat of the cells to be isolated, to facilitate subsequent pelleting and quantification of cells present in d e tube. An aliquot containing 3 x IO 7 PBPC cells, isolated as described in Example 1, was added to each tube, and initial cell number was noted. The cells were incubated for 30 minutes at room temperature, with gentle mixing at 15 minute intervals. The cell mixture was then centri¬ fuged at 550 x g for 10 minutes. Following centrifugation, die supernatant was discarded and d e cell pellet was re-suspended in 5 ml of Iscove's medium supplemented widi 10% fetal calf serum. The cell mixture was then incubated for 24 hours at room temperature, and then tested in die CFU assay described in Example 6. Results of this experiment are shown in FIG. 1. Example 9 Sterile Centrifugation Kit A sterile centrifuge kit was prepared for separating CD34 + hematopoietic progenitor cells from peripheral blood mononuclear ceils (PBPC) as follows: an OCS particle-based density particle density gradient suspension was prepared as described in Example 1. A stock solution was prepared by mixing 12 parts of the OCS particle suspension with 1 part of 10 x calcium and magnesium-free phosphate buffered saline (PBS) and concentrated PVP (PVP-10; Sigma) to prepare a medium having a density of 1.0605 + . 0.0005 gr/ml, pH 7.4, an osmolality 280 mOsm/Kg H 2 0, and a concentration of 4% PVP. The density of the OCS particle solution may be adjusted on a densitometer to precisely define its accuracy up to at least e fourth decimal place. Fifteen milliliters of diis suspension was added to a polypropylene 50 ml conical centrifuge tube (Baxter). The tube was sealed widi the standard screw cap supplied widi die tube. The tube and its contents were then exposed to gamma irradiation 2.5-3.5 megaRads by placing it in a chamber. The resulting sterile centrifuge kit was used in separating CD34 + cells from PBPC, as described in Example 4 herein. Example 10 Enrichment of CD34 + Cells from Cell Mixtures A. Preparation of Density Gradients A.l. PERCOLL Density Gradient "PERCOLL" solution was purchased from Pharmacia Biotech (Uppsala, Sweden) and stored at 4 C C according to die recommendation of die vendor. A stock solution was prepared by mixing 12 parts of "PERCOLL" with 1 part of 10 x calcium and magnesium-free phosphate buffered saline (PBS). The pH of the solution was adjusted to 7.4 and die osmolality to 280 mOsm/Kg H 2 0. For use in separating CD34 + cells in a cell mixture, die stock solution was further diluted widi calcium and magnesium-free PBS to a density of 1.0605 ± 0.0005 gr/ml and used at room temperature. It was crucial to adjust d e density of die gradient to an accuracy of widiin ± 0.0005 gr/ml of 1.0605 gr/ml in order to ensure reproducibility and accuracy of cell separation. This was done by a high precision digital density meter such as DMA 48 (Anton PAAR USA, Ashland, VA). All procedures were performed under sterile conditions and at room temperature. A.2. Organosiianized Colloidal Silica (OCS) Density Gradient An OCS particle suspension is an organosiianized colloidal silica particulate suspension prepared as described by Dorn (U.S. Patent 4,927,749). OCS particles ranging in size from 3-22 nm, and particularly from 13-18 nm, form a stable suspension at acts as a density gradient medium for separating heterogeneous mixtures of cells on die basis of buoyant density. Unless otiierwise indicated, die OCS particle suspension was prepared in a physiological saline solution to have a density of 1.0605 gr/ml at an osmolality of 280 mOsm/kg H 2 0, as defined for die enrichment of CD34 + cells from PBSC samples. The suspension was produced under GMP conditions and is sterilized by gamma irradiation in die presence of at least 0.05% poly¬ vinylpyrrolidone (PVP-10; Sigma Chemical Co., St. Louis, MO). The PVP was added as a dry powder to a suspension of OCS particles pre-adjusted to the approximate final desired specific density. PVP concentrations of at least 0.05% and as high as 10% are preferred for use in the methods, apparatus and kit of the invention. B. Density Gradient Centrifugation of Blood and Bone Marrow Buffy Coats Apheresed peripheral blood was applied directly onto die density gradient. However, unless otiierwise indicated, complete blood and bone marrow aspirates were processed to a buffy coat (removal of red cells), according to standard mediods, before tiiey were applied onto die density gradient. Apheresed blood samples were layered on a "PERCOLL" gradient previously adjusted to a density of 1.0605 ± 0.0005 gr/ml, an osmolality of 280 mOsm/Kg H 2 0, and a pH of 7.4 in a 50 ml conical cell-trap tube or a commercially available tube. The cell-trap tube contained a constriction in a location so that approximately 15ml of "PERCOLL" was in d e lower compartment and 5ml of "PERCOLL" was above die constriction. It was critical to completely fill die volume under d e constriction witii "PERCOLL" to prevent die formation of air bubbles. Generally, 20ml of apheresed blood samples were layered on top of diis gradient. The tube was centrifuged at 850 x g for 30 minutes at room temperature. The cells lodged at me interface of the gradient; i.e. , on top of "PERCOLL," were collected by pouring the entire content of die upper compartment of the tube into anodier 50 ml tube. The cell pellet in the region below die constriction were prevented from pouring off when the tube was inverted. In order to compare the cell separation method described in d e preceding paragraph wid conventional mediods of separation, die test samples were also was layered on "FICOLL- HYPAQUE" (Pharmacia). The density of the stock "FICOLL" solution was at 1.077 ± 0.001 gr/ml and the osmolality at 320 mOsm/kg H 2 0 as published by the vendor. Bone marrow cells were prepared as described in Example 2. C. Density Adjusted Cell Sorting fDACS Cl. Preparation of Carrier Particles Silica beads (1.4 microns) obtained from Bangs Laboratories, Carmel, IN were washed widi concentrated HCl for 2 hours at room temperature and vortexed intensely every 15 minutes to break up bead clumps. After washing, the beads were centrifuged at 850 g for 5 minutes. The HCl containing supernatant was decanted and the beads were washed wid deionized H 2 0 wid intensive vortexing to break up die clumps. The beads were incubated at room temperature overnight in concentrated nitric acid widi constant stirring using a magnetic stirrer. The beads were dien centrifuged at 850 g for 5 minutes and washed 3 times widi deionized water, using 50 ml of deionized H 2 0 at each step. The beads were vortexed intensely in between each wash to avoid bead clumping. To prevent microbacterial contamination, the beads were stored at 0-4 degrees centigrade in deionized H 2 0 until further use. To silanize the beads , eitiier 3-aminopropy ltriethoxysilane , (3 -iodopropyl)trimedιoxysilane or [l-9trimethoxysilyl)-2(m-(or p) chloromethyl)phenyl] ediane were used. Forty mis of silane solution (a 10% solution in 95% ethanol/deionized H 2 0) was added per 4 gr of beads. The bead mixture was rotated end over end for 1 hour at room temperature. The beads were centrifuged at 850 g for 5 minutes and d e excess silane was washed off using 95% ethanol/deionized H 2 0 in a volume of 100 ml. The beads were vortexed intensely in between each wash step to avoid bead clumping. After die washing step, die beads can be dried and stored. Alternatively die beads can be stored in 95% ethanol/deionized H 2 0 in die cold which prevents clumping of the beads. The silanized beads were incubated overnight in 2.5% glutaraldehyde at room temperature. The next day, the beads were centrifuged at 850 g for 5 minutes and the free glutaraldehyde was washed off widi deionized H 2 0 in a volume of 100 ml per 5 gr beads. The beads were vortexed intensely in between each wash step to avoid bead clumping. The antibody was added to die aminopropyl beads in an excess, at least 3 mg/m 2 total bead surface and rotated end over end overnight at room temperature. The next day, the beads were centrifuged at 850 g for 5 minutes and the free protein was washed off widi 100 ml of deionized H 2 0. The beads were vortexed intensely in between each wash step to avoid bead clumping. The beads were stored in deionized H 2 0 containing 0.1 sodium azide in d e cold. The final bead suspension should contain 70-90% single beads and 10-30% predominantly duplet and triplet beads. The binding efficiency of the antibody conjugated beads (in terms of the percent of beads d at are coated) can be determined using flow cytometric analysis and a fluoresceinated antibody directed to die coupled antibody. Alternatively, the antibody may be added to die silanized beads directly widiout die glutaraldehyde linking. C.2. Anti-CD45-coated Beads Apheresed blood product was incubated with 1.4μ aminopropyl glass beads (Bangs Laboratories Inc., Carmel, IN) that were glutaraldehyde coated widi an anti-CD45 antibody (clone ALB-12, Biodesign International, Kennebunk, ME) for 45 minutes at room temperature. The entire blood cell mixture was layered on OCS + 0.5% PVP (1.0605 ± 0.0005 gr/ml, 280 mOsm/Kg H 2 0, pH 7.4) in a 50 ml tube. C.3. Goat Anti-Mouse IgG (GAM-IgG)-coated Beads Cells from d e PBSC sample were counted by trypan blue exclusion, washed and re- suspended in PBS supplemented with 0.1 % bovine serum albumen (BSA) at a concentration of IO 7 cells/ml. Anti-CD3, CD4 and CD8 mAbs were added to give a final concentration of 3-10 μg/ml. The cells were incubated for 30 minutes on ice and d en centrifuged. The cell pellet was re-suspended in 0.1% BSA in PBS at a cell concentration of IO 7 cells/ml. Goat anti- mouse IgG (GAM-IgG) coated DACS beads were counted and added to the cells to give bead ell ratios of between 4: 1 and 8: 1. The beads and calls were incubated for 15 minutes. During ie incubation die cell:bead mixture was gently re-suspended every 5 minutes. The cell-bead mixture was then layered onto OCS density gradient suspension and centrifuged at room temperature at 850 x g for 30 minutes. The cells at die interface were harvested by decanting die centrifugation tube, counted and prepared for FACS analysis. To examine non¬ specific depletion, a similar procedure was used but the incubation with the monoclonal antibodies was omitted. Experiments to examine die efficacy of DACS for T lymphocyte purging were performed using mobilized PBSC preparations and anti-CD3, CD4 and CD8 mAbs. The yield of T lymphocytes was monitored using an anti-CD2 mAb in order to avoid artifacts associated widi mAbs recognizing die same epitopes on d e differentiation antigens. D. Conjugation of Cell Attachment Molecules to Carrier Particles D. l. Monoclonal Antibodies Phycoerydirin-conjugated (PE-CD34) anti-CD34 monoclonal antibodies (hematopoietic progenitor cell marker) and fluorescein isothiocyanate-conjugated (FITC-CD45) anti-CD45 monoclonal antibodies (pan-leukocyte marker) were obtained from Becton Dickinson, Inc. (San Jose, CA). Unconjugated antibodies directed to CD45, CD16 (granulocytes, monocytes), CD3 (T lymphocytes), CD 14 (monocytes) were prepared in the laboratory, according to mediods well known in die art. The anti-CD3 (clone Leu4), CD4 and CD8 mAbs were obtained from Dr. E. Engleman of Stanford University Hospital Blood Bank. In addition, FITC-CD2, PE-CD34 and FITC- CD45 were purchased from die same vendor. Antibodies were conjugated to eidier goat anti-mouse coated magnetic beads or to goat anti-mouse coated aminopropyl glass beads by overnight incubation at room temperature. Alternatively, die antibodies could be bound directly to diese beads widiout die goat anti-mouse bridge or could be bound via an avidin-biotin coupling reaction. In addition die antibodies could be cleaved into Fab2 fragments in order to reduce non-specific binding of cells to die beads via dieir Fc portion. D.2. Goat Anti-mouse IgG (GAM-IgG) Mouse IgGl conjugated with eidier FITC ar PE were purchased from Becton Dickinson (San Jose, CA) and used as isotype control. Several different procedures were used to prepare DACS beads coated widi goat anti- mouse IgG (GAM-IgG) (Biodesign International). The goal was to maximize specific depletion and minimize non-specific depletion during DACS. DACS beads were covalently coated eidier with GAM-IgG or F(ab) 2 GAM-IgG (East Coast Biologicals) in a random orientation or GAM-IgG in a specific orientation to maximize the availability ofthe Ig binding sites to attach to antigen. In addition, some preparations of beads were coated widi polyediylene glycol (PEG) (Shearwater) to reduce die non-specific affinity of die beads for protein or cells. The different chemistries are outlined below. Random GAM-IgG or F(ab) 2 GAM-IgG Silica beads were coated with aminopropyltriethoxysilane (Sigma) which has -NH 2 groups available to be conjugated to d e -CHO of glutaraldehyde (Sigma). According to die chemistry employed, glutaraldehyde dien forms a bridge and binds to die -NH 2 group on lysines diroughout die GAM-IgG or F(ab) 2 GAM-IgG molecules, when both antibody and glutaralde¬ hyde were added to die preparation. The orientation of die protein molecule relative to die bead was considered to be random, on die grounds tiiat any lysine with an -NH 2 side-chain will potentially conjugate to die -CHO of the glutaraldehyde. This gives an ATES-GLUT— IgG chain with die IgG or IgG-F(ab) 2 in a random orientation. GAM-IgG in a specific orientation Silica beads were coated widi aminopropyltrietiioxysilane having -NH 2 groups available to be conjugated to die -CHO group of the carbohydrate on die glycosylated region of the Fc portion of the GAM-IgG molecule. The GAM-IgG molecule is only glycosylated on die Fc portion so diat linking GAM-IgG to die bead surface dirough diis region will have the effect of orienting die IgG molecule with d e Fc portion next to the bead surface and die IgG binding sites away from die surface. This treatment dieoretically increases d e number of binding sites available to bind antigen and decreases die availability of the FC portion to bind Fc receptors. This gives an ATES-CHO-IgG chain widi die IgG in a specific orientation. PEG-aldehyde random GAM-IgG Silica beads were coated witii aminopropyltrietiioxysilane which has -NH 2 groups available to be conjugated to one -CHO group of an aldehyde-PEG-aldehyde molecule where PEG is sandwiched between two aldehyde residues. GAM-IgG dien bound to die odier -CHO residue to give an ATES-CHO-PEG-CHO-IGg chain widi the IgG in a random orientation. E. Analysis Antibody staining, FACS analysis and CF assays were carried out as described in Examples 5 and 6. F. Long-Term Culture Initiating Cell (LTC-IC) Assay /Functional Determination of Uncommitted CD34 + Cells The number of uncommitted hematopoietic progenitor cells in a cell mixture was determined by d e long term culture initiating culture. The cells were seeded on an irradiated stroma feeder layer and a determination of CFU's was made in function of time. Hematopoiet¬ ic stem cells were able to self-renew and gave rise to CFU's in this system for a period that exceeded 5 weeks. Long term bone marrow stromal cultures were initiated in 96 well plates (IO 6 cells in 200 μl per well) in α-MEM medium supplemented witii 12.5% horse serum, 12.5% fetal calf serum, 2 mM L-glutamine, 0.2 mM i-inositol, 20 μM folic acid, 10"* M 2- mercaptoemanol and were kept at 33 °C in a humidified atmosphere. At weekly intervals, half die medium was removed and replaced by an equal volume of fresh medium. After 2 weeks of culture, die confluent stroma layers were gamma irradiated (2000 rad) to kill endogenous hematopoietic cells. Unfractionated samples and cell preparations after separation were seeded onto die irradiated stroma layers in die same medium supplemented with lO^M hydrocortisone. After five weeks of culture die adherent and non-adherent cells were collected and screened in die CFU assay as in Part G, above. G. Natural Killer (NK) Cell Assay K562 target cells were labeled with 100 μCi 51 Cr for 1 hour at 37 " C and then washed four times and counted. The target cells were co-cultured for 4 hours in V-bottom 96 well multiwell plates with unfractionated apheresed blood and cells from the different fractions after cell separation. Effector and target cells were mixed at different ratios, 100: 1, 50:1, 25: 1 and 12:1. For example, the 100: 1 ratio contained 5xl0 5 effector cells and 5xl0 3 target cells. After die incubation period, lOOμl of d e supernatant was harvested and counted in a scintillation counter. Maximal and spontaneous 51 Cr release was measured counting eidier 50μl of the stock target solution and supernatant from die effectors by tiiemselves, respectively. The percent cytotoxicity was determined according to formula: Percent Cvto t oxicitv = cpm experiment - cpm spontaneous release y y cpm maximal release - cpm spontaneous release J. Mixed Lymphocyte Culture and Natural Suppressor Cell Activity Cells from two different buffy coats were mixed in a flat bottom 96 well multiwell plate at 10 5 cells of each. One of the buffy coats received 3000 rad and was referred to as the "stimulators". The otiier buffy coat was used untreated and referred to as "responders." Unfractionated apheresed peripheral blood products (APBL) or cells from die different density fractions were added to diese co-cultures at IO 5 cells per well. These cells were referred to as "suppressors" and received 1500 rad prior to being added to die MLR. The cells were cultured for 5 days and dien pulsed with [ 3 H]-thymidine (1 μCi/well). 18 hours later, the cells were harvested and die amount of diymidine incoφorated determined in a scintillation counter. The percent suppression induced by the suppressor cells was determined by die formula: Percent Suppression = cpm control - cpm experiment ^ r cpm experiment Example 11 Purging Tumor Cells from Peripheral Blood The human B-cell lymphoma cell line SU-DHL4 (Stanford University Blood Bank, Stanford, California) was used as a standard experimental model to model tumor cell contamination of peripheral blood. This cell line contains a reciprocal translocation between die long arms of human chromosomes 14 and 18, resulting in a juxtaposition of the bcl-2 proto-oncogene to die joining region of die immunoglobulin (Ig) heavy chain gene. This translocation, which leads to elevated levels of the bcl-2 gene product, is found in 85% of follicular small cleaved cell lymphomas and 25% of diffuse large cell lymphomas. The translocation breakpoint in the bcl-2 gene is located in either the major breakpoint region (MBR) spanning approximately 150 bp in the 3' untranslated region (UTR) of die bcl-2 gene or e 500 bp minor cluster region (MCR) located approximately 20 kb downstream of die bcl-2 gene. Using translocation-specific primer pairs and quantitative PCR by limiting dilution analysis, depletion of SU-DHL4 cells in DACS purging experiments can be monitored. The human SK-BR3 adenocarcinoma cell line (American Type Culture Collection (ATTC), Rockville, MD) was used as a standard experimental model to model breast cancer cell infiltration of peripheral blood. These cells react witii die mouse anti-human Her2/Neu monoclonal antibody, indicating they possess a Her2/Neu antigen. Prior to being spiked into normal buffy coats, the SK-BR3 cells were pre-loaded with calcein AM, a fluorogenic substrate so mat they can be enumerated by FACS analysis. Alternatively or in addition breast tumor cells contain estrogen receptors and react widi antibodies to such receptors and otiier receptor binding molecules, such as estrogen and estrogen agonists. For puφoses of die present invention, such cells can be captured, as in die tumor cell purging mediods described herein, by any molecule that binds widi sufficient affinity to such a tumor cell antigen, including, but not limited to antibodies, estrogen- or estrogen agonists. In this context, a sufficient affinity of binding is an affinity approximately equal to or widiin 2 log binding affinity units of me mouse anti-human Her2/Neu monoclonal antibody (HB 8694, 520C9; ATCC, Rockville, MD) PCR Analysis High molecular weight genomic DNA was prepared using die QIAamp Blood Kit (Qiagen, Chatsworth, Ca.) and quantitated by optical density at 260nm. PCR reactions were done using a Perkin-Elmer model 9600 tiiermocycler for 35 cycles. Each amplification cycle consisted of 94°C (15"), 55°C (60"), and72°C (15"), with a 72°C ( 6 min.) extension after the final cycle. For nested reactions, 1 ml of die first-round reaction was used as template. The following oligonucleotides were used for analysis of samples: (i)5-CAGCCTTGAAACATT- GATGG-3 (MBR) (ii)5-ACCTGAGGAGACGGTGACC-3 (J „ Consensus) (iii)5-ATGCTGT- GGTTG ATATTTCG-3 (MBR nested) (iv)5-ACCTGAGG AGACGGTGACC-3 (J H nested Con- sensus). Amplification products were visualized by agarose gel electrophoresis. FACS analysis SK-BR3 cells were pre-loaded by incubating diem in calcein AM and dien seeded into normal peripheral blood mononuclear cells at a final frequency of 2% . DACS was performed as described below using die mouse monoclonal antibody to human Her-2/Neu. Aliquots of die experimental samples were analyzed by FACS analysis using die LYSYS II program. For the analysis 100,000 events were collected for each sample. Density Adjusted Cell Sorting Peripheral blood progenitor cells (PBPC) and buffy coats were obtained from the Bone Marrow Transplantation Laboratory at the Stanford University School of Medicine, Palo Alto, Ca and from the Stanford University Blood Bank, Palo Alto, CA. Monoclonal antibodies to CD9, CD10, CD19 and CD20 were obtained from Caltag Laboratories (So. San Francisco, CA). The anti-CD20 mAb (clone IF5) and the anti-human Her2/Neu mAb (HB 8694, 520C9) and the SK-BR3 adenocarcinoma cell line were obtained from the ATCC (Rockville, MD). Fluorescein isodiiocyanate (FITC) conjugated mouse IgGl and mouse anti-human CD45 as well as phycoerythrin (PE) conjugated mouse IgGl and mouse anti-human CD34 antibodies were obtained from Becton Dickinson, Ine (San Jose, Ca.). The FACS procedure and flow cytometric analysis of samples was performed as described above. Purging and Electrophoretic Analysis of Peripheral Blood Cells A Peripheral blood buffy coat, spiked (5%) widi die t(14;18)+ B cell lymphoma cell line SUDHL-4, was incubated with an anti-B cell mAb cocktail specific for CD9, CD 10, CD 19 and CD20 cells. After washing, the cell mixture was incubated widi GAM-IgG coated DACS beads at a 8:1 bead:cell ratio and then loaded onto OCS suspension at densities ranging from 1.0605 to 1.0720, with comparable results. Following centrifugation for 30 minutes at 850g, DNA was prepared from the cells at die interface and analyzed by standard gel electrophoretic methods at different concentrations (lmg, O.lmg, O.Olmg, O.OOlmg) by two rounds of PCR using t(14;18) specific primers. The number of t(14;18) translocations was furthermore determined by limiting dilution PCR. As control, die same protocol was used on un-spiked cells, SUDHL-4 spiked sample and SUDHL-4 spiked sample treated widi DACS beads witiiout prior incubation widi die anti-B cell mAbs cocktail. Centrifugation of d e cell mixture over OCS suspension did not result in a reduction of die PCR signal. Whereas the DACS beads alone induced a small reduction of the PCR signal, die PCR signal was completely absent in the SUDHL-4 spiked sample that was treated widi botii die anti-B ceil mAb cocktail and die DACS beads. The determination of the number of t(14;18) translocations by limiting dilution PCR, showed that in die latter experimental group the number of SUDHL-4 cells was reduced by 2-3 logs (100- 1000-fold). Preliminary experiments indicate that an additional round of DACS in this context further decreases the number of SUDHL-4 cells by approximately an additional log. Example 12 Isolation of Dendritic Cells Buffy coats prepared from one unit of blood from HLA-A0201 positive volunteer healthy donors are obtained from die Stanford University Blood Center (Stanford, CA). Cells are harvested from the leukopacs, diluted to 60 mL using Ca + + /Mg + + free phosphate buffered saline (D-PBS; Gibco Laboratories, Grand Island, NY) and layered over two 15 mL columns of OCS separation medium (density 1.0720 gr/ml, pH 7.4, 280 mOsm/kg H 2 0) in 50 mL centrifuge tubes, preferably cell-trap tubes. The tubes are centrifuged at 1000 x g for 35 minutes at room temperature. The centrifuge run is allowed to stop witiiout braking and die peripheral blood mononuclear cells (PBMC), present at the interface, are harvested. PBMC are re-suspended in D-PBS, centrifuged once at 650 x g for 10 minutes and twice more at 200 x g for 5 minutes to remove platelets. Platelet-depleted PBMC are re-suspended in 60 mL of D-PBS, layered on top of two columns of 15 mL of OCS (density 1.0610 gr/ml, 280 mOsm/kg H 2 0) in a cell-trap tube of the present invention and centrifuged at 650 x g for 25 minutes at 4°C witiiout braking. The resulting interface (primarily monocytes) and pellet cells (primarily lymphocytes) are harvested and washed wid D-PBS by centrifugation at room temperature (once at 650 x g for 10 minutes and twice thereafter at 200 x g for 5 minutes). In instances where the dendritic cells are used to generate peptide-specific cytotoxic T lymphocytes (CTL) for puφoses of elucidating their antigen presentation function, the interface fraction (mostly monocytes) is re-suspended in cold pooled human AB serum (Irvine Scientific, Santa Ana, CA) to which an equal volume of 80% AB serum 20% dimediyl sulfoxide (DMSO) (Sigma Chemical Company, St. Louis, MO) is added dropwise. The resulting cell suspension is aliquoted into cryovials and frozen in liquid nitrogen. The monocytes can be used for re- stimulation of CTL for expansion. The pellet fraction is re-suspended in 100 mL of AB Culture Medium, inoculated into two T-75 tissue culture flasks and cultured in a humidified 5% C0 2 incubator for 40 hours. Following die incubation, die non adherent cells are harvested by moderate pipeting, washed and re-suspended at a concentration of 2 - 5 x IO 6 cells/mL in AB Culture Medium. The cell suspension is overlayered over four columns of 4.0 mL OCS separation medium (density 1.0565 gr/ml, pH 7.4, 280 mOsm/kg H 2 0), in AB Culture Medium and centrifuged at 650 x g for 20 minutes at room temperature witiiout braking. The interface and pellet cells are harvested and washed in AB Culture Medium (Basal RPMI-1640 medium, Gibco Laboratories, Grand Island, NY) by centrifugation once at 650 x g for 10 minutes and twice tiiereafter at 200 x g for 5 minutes each at room temperature. The yield and viability of both cell fractions is estimated by counting on a hemocytometer using trypan blue exclusion. The purity of dendritic cells in the interface fraction is quantified following analysis on a flow cytometer (FACS). Dendritic cells are characterized as negative for cell phenotype markers CD3 (T lymphocytes), CD14 (monocytes), CD16 (NK cells) and CD20 (B-cells) and positive for HLA class II expression using dual staining widi HLA-DR (on die FITC channel) and a cocktail of CD3, CDK, CD 16, CD20 (on the PE channel). Dual staining widi IgG2a on botii the FITC and PE channels can be used as isotype control. The moφhology of d e cells can also be evaluated using photomicroscopy. The DC enriched fraction contains large sized veiled cells widi cytoplasmic processes extending from e cell surface, features characteristic of DC.

Claims: IT IS CLAIMED: 1. A composition for use in cell separation, comprising a silanized colloidal silica particle-based cell separation medium containing at least about 0.05% polylactam. 2. The composition of claim 1 , wherein said colloidal silica particle-based cell separation medium is composed of organosiianized silica particles. 3. The composition of either of claims 1 or 2, wherein said separation medium containing said polylactam is sterilized. 4. The composition of claim 3, wherein said composition is sterilized by autoclaving or by ionizing radiation. 5. The composition of claim 4, wherein said ionizing radiation is selected from gamma irradiation and E-beam irradiation. 6. The composition of any of claims 1 to 5, wherein said polylactam is polyvinylpyrrolidone present at a concentration between about 0.1 and about 10 percent. 7. The composition of any of claims 1 to 6, wherein said particle is a microsphere having a diameter of between about 0.003 and 50 microns. 8. The composition of any of claims 1 to 7, for use in isolating a selected rare blood cell from a cell mixture, wherein said silanized colloidal silica particle suspension has a specific density widiin about 0.0005 gr/ml of said selected cell. 9. The composition of claim 8, wherein the rare blood cell is selected from the group consisting of hematopoietic progenitor CD34 + cells, dendritic cells, natural killer cells, natural suppressor cells, cytotoxic T cells, tumor cells and nucleated fetal cells. 10. The composition of claim 9, wherein said rare blood cell is a hematopoietic progenitor CD34 + cell, said cell mixture comprises peripheral blood mononuclear cells, and said organosiianized colloidal silica particle suspension has a specific density of 1.0605 gr/ml. 11. The composition of claim 9, wherein said rare blood cell is a hematopoietic progenitor CD34 + cell, said cell mixture consists of bone marrow cells, and said organosiianized colloidal silica particle suspension has a specific density of 1.0685 gr/ml. 12. The composition of claim 9, wherein said rare blood cell is a dendritic cell, said cell mixture comprises peripheral blood mononuclear cells, and said organosiianized colloidal silica particle suspension has a specific density selected from the group consisting of 1.0770 gr/ml, 1.0720 gr/ml, 1.0650 gr/ml, 1.0610 gr/ml, and 1.0565 gr/ml. 13. A kit for cell separation, comprising a centrifugable container, and a silanized colloidal silica particle-based cell separation suspension according to any of claims 1 to 12. 14. A mediod of sterilizing a silanized silica particle-based cell separation medium, comprising subjecting said separation medium to autoclaving or ionizing radiation in die presence of at least about 0.05 percent polylactam. 15. The method of claim 14, wherein said polylactam is polyvinylpyrrolidone present at a concentration between about 0.1 and about 10 percent. 16. The mediod of eitiier of claims 14 or 15, wherein said silica particle is an organosiianized colloidal silica particle. 17. A method of isolating a selected rare blood cell from a cell mixture, comprising layering die mixture containing said rare hematopoietic cell on a silanized colloidal silica particle-based cell separation medium according to any of claims 1 to 7, and centrifuging said mixture at a centrifugal force sufficient to pellet cells having a specific density that is greater man die specific density of the selected cell. 18. The mediod of claim 17 wherein said mixture containing said rare cell and said cell separation medium are contained in a centrifugation tube having side walls and a closed bottom, a constriction member disposed widiin die tube, said constriction member positioned and constructed to retain fluid in die bottom portion of d e tube below d e constriction member, when the tube is inverted, and wherein said cell-separation medium has a specific density diat is widiin ^0.0005 gr/ml of the specific density of die selected cell type and said medium is contained in the bottom portion of the tube and extending above said constriction member to a level above an opening formed by said constriction member, such at cells which are captured at an interface between die cell-separation medium and a lower-density medium, after centrifugation, are discharged widi the lower-density medium when the tube is inverted. 19. The mediod of claim 18, wherein said tube includes a closed top, a first port in said top through which fluid material can be introduced into die tube, a second port in said top, and a closed fluid channel communicating the second port wid die bottom of die tube below said constriction member. 20. The method of claim 17, wherein die selected rare cell is a functional hematopoietic progenitor cell, die cell mixture is a peripheral blood mononuclear cell mixture, and the specific density of the cell separation density gradient suspension is 1.0605 gr/ml. 21. The method of claim 17, wherein the selected rare cell is a functional hematopoietic progenitor cell, the cell mixture is a bone marrow cell mixture, and die specific density of the cell separation density gradient suspension is 1.0685 gr/ml. 22. The mediod of claim 17, wherein die selected rare blood cell is a tumor cell, the cell mixture is derived from blood or bone marrow, and wherein die cell separation medium has a specific density selected from die range 1.0490-1.0580 gr/ml. 23. The method of claim 17, wherein the selected rare blood cell is a dendritic cell, and wherein said cell separation medium has a specific density selected from die group consisting of 1.0770 gr/ml, 1.0720 gr/ml, 1.0650 gr/ml, 1.0610 gr/ml, and 1.0565 gr/ml. 24. The method of claim 17, wherein the selected rare blood cell is a cytotoxic T lymphocyte and wherein said cell separation medium has a specific density selected from die group consisting of 1.0720 gr/ml, 1.0610 gr/ml and 1.0565 gr/ml. 25. The method of any of claims 17 to 24, which further includes adding to die cell mixture a suspension which includes a silanized silica particle to which is attached a cell antigen-specific binding molecule. 26. The method of claim 25, for use in depleting T-cell lymphocytes, wherein said cell antigen-specific binding molecule binds to an antigen present on said T-cell lymphocytes. 27. The mediod of claim 26, wherein said cell antigen-specific binding molecule is selected from d e group consisting of anti-CD3, anti-CD4 and anti-CD8 mouse monoclonal antibodies.

Inventors: PALATHUMPAT VARGHESE, VLASSELAER PETER VAN, PALATHUMPAT, VARGHESE, VAN VLASSELAER, PETER
Applicants: ACTIVATED CELL THERAPY INC, ACTIVATED CELL THERAPY, INC.
Application year: 1996
Main classification: B01J20
IPC codes: B01D15, B01J20, B01L3, C12N5, C12N15, C12P21, G01N33