METHODS FOR CALCIUM PHOSPHATE TRANSFECTION

Abstract: A method is provided for calcium phosphate transfection of a eukaryotic host cell wherein particles comprising calcium phosphate and a desired nucleic acid are grown to an optimal size and then contacted with the host cell under conditions that substantially reduce the particle growth rate and allow the host cell to take up the particles to form a transfected cell.

Description: METHODS FOR CALCIUM PHOSPHATE TRANSFECTION FIELD OF THE INVENTION This invention relates to the field of nucleic acid transfection, and more particularly to methods for nucleic acid transfection of eukaryotic cells by calcium phosphate co-precipitation DESCRIPTION OF THE BACKGROUND AND RELATED ART The ability to introduce foreign DNA into eukaryotic host cells is one of the principle tools of recombinant DNA technology Methods for transfectmg eukaryotic host cells with foreign DNA can be broadly grouped into four categories (1) direct introduction of cloned DNA by microinjection or microparticle bombardment, (2) use of viral vectors, (3) encapsulation within a earner system, and (4) use of facilitators such as calcium phosphate and diethylaminoethyl (DEAE)-dextran Of the reagents used as facilitators of DNA transfection, calcium phosphate remains the most widely used because of its simplicity and general effectiveness for a wide variety of cell types The oπginal protocol for calcium phosphate transfection was described by Graham and van der Eb, Virology, 52 456-467 (1973) This method was modified by Wigler et al , Proc Natl Acad Set , 76 1373- 1376 (1979) and by Chen and Okayama, Mol Cell Btol , 7 2745-2752 (1987) Nevertheless, the original and modified protocols yield relatively low transfection efficiencies and expression in experiments geared towards transient or stable DNA transfer Accordingly, there is still a need for an improved method of calcium phosphate transfection In addition, there is a need for methods of calcium phosphate transfection in suspension culture, particularly m the area of large scale suspension culture This and other objects will be apparent to those skilled m the art SUMMARY OF THE INVENTION The invention provides for a method for introducing a desired nucleic acid into a eukaryotic host cell compπsing admixing Ca 2+ , P0 4 " and the desired nucleic acid to form a precipitation mixture, incubating the precipitation mixture to form particles comprising calcium phosphate and the desired nucleic acid, and allowing the particles to grow to an average length of up to about 300 nanometers (nm), performing a step selected from the group consisting of (1) diluting the precipitation mixture and simultaneously admixing the precipitation mixture to a eukaryotic host cell lacking a cell wall to form a transfection mixture wherein the particles are capable of further growth at a rate that is substantially lower than the rate at which the particles grew in the precipitation mixture, and (2) diluting the precipitation mixture to form a diluted precipitation mixture, and thereafter admixing the diluted precipitation mixture to a eukaryotic host cell lacking a cell wall to form a transfection mixture wherein the particles are capable of further growth at a rate that is substantially lower than the rate at which the particles grew in the precipitation mixture, and incubating the transfection mixture to allow the eukaryotic host cell to take up the particles to form a transfected cell The invention further provides a method for introducing a desired nucleic acid mto a eukaryotic host cell compπsing admixing Ca 2 \ P0 4 and the desired nucleic acid to form a precipitation mixture, incubating the precipitation mixture for a period of up to about 60 seconds to form a precipitate compπsing calcium phosphate and the desired nucleic acid, performing a step selected from the group consisting of (1 ) diluting the precipitation mixture and simultaneously admixing the precipitation mixture to a eukaryotic host cell lacking a cell wall to form a transfection mixture wherem the particles are capable of further growth at a rate that is substantially lower than the rate at which the particles grew in the precipitaUon mixture, and (2) diluting the precipitation mixture to form a diluted precipitation mixture, and thereafter admixing the diluted precipitation mixture to a eukaryotic host cell lacking a cell wall to form a transfection mixture wherem the particles are capable of further growth at a rate that is substantially lower than the rate at which the particles grew m the precipitation mixture, and mcubatmg the transfection mixture to allow the eukaryotic host cell to take up the precipitate to form a transfected cell The invention also provides for a method for introducing a desired nucleic acid into a eukaryotic host cell in suspension comprising admixing Ca + , P0 4 ' and the desired nucleic acid to form a precipitation mixture, incubating the precipitation mixture to form particles comprising calcium phosphate and the desired nucleic acid, and allowing the particles to grow to an average length of up to about 300 nm, performing a step selected from the group consisting of (1) diluting the precipitation mixture and simultaneously admixing the precipitation mixture to a mixture comprising in suspension a eukaryotic host cell lacking a cell wall to form a transfection mixture wherem the particles are capable of further growth at a rate that is substantially lower than the rate at which the particles grew in the precipitation mixture, and (2) diluting the precipitation mixture to form a diluted precipitation mixture, and thereafter admixing the diluted precipitation mixture to a mixture compπsing in suspension a eukaryotic host cell lacking a cell wall to form a transfection mixture wherem the particles are capable of further growth at a rate that is substantially lower than the rate at which the particles grew in the precipitation mixture, and mcubatmg the transfection mixture to allow the eukaryotic host cell to take up the particles to form a transfected cell m suspension BRIEF DESCRIPTION OF THE FIGURES Fig 1 is a graph depicting the effect of temperature and mcubation time on the DNA binding capacity of a calcium phosphate precipitate formed with 25 micrograms per millihter (μg/ml) DNA, 125 millimoles per liter (mM) Ca + and 06 mM P0 4 ' Shaded, open and closed columns represent mcubation times of 1 , 5 and 20 minutes, respectively Fig 2 is a graph depicting the effect of DNA concentration and mcubation time on the DNA binding capacity of a calcium phosphate precipitate formed with 125 mM Ca 2+ and 0 75 P0 4 3 Shaded, open and closed columns represent mcubation times of 1 , 5 and 20 minutes, respectively Fig 3 is a graph depicting the effect of Ca 2+ concentration, mcubation temperature and mcubation time on the DNA binding capacity of a calcium phosphate precipitate formed with 50 μg ml DNA and 0 75 mM P0 4 3' Shaded, open and closed columns represent mcubation times of 1 , 5 and 20 minutes, respectively Fig 4 is a graph depicting the effect of DNA-calcium phosphate co-precipitation mcubation time on the transient expression of β-galactosidase in transfected CHO or 293 host cells as determined by the number of host cells that are positive for 5-bromo-4-chloro-3-ιndolyl-β-D-galactosιde (X-gal) stiumng as a percentage of the total population of cells The DNA-calcium phosphate co-precipitation was earned out with 25 μg/ml DNA, 125 mM Ca 2+ , and 0 75 mM P0 4 3' Shaded and closed columns represent Chinese hamster ovary (CHO) cell and human embryonic kidney 293 cell (293 cell) hosts, respectively Fig 5 is a graph depicting the effect of DNA-calcium phosphate co-precipitation mcubation time on the transient expression of human tissue plasmmogen activator (tPA) in transfected CHO or 293 cells as determined by the quantity of tPA detected in the cell culture supernatant by ELISA The DNA-calcium phosphate co-precipitation was earned out with 25 μg/ml DNA, 125 mM Ca 2" ' ' , and 0 75 mM P0 4 3 Shaded and closed columns represent CHO and 293 cell hosts, respectively Fig 6 is a graph depicting the effect of DNA-calcium phosphate co-precipitation mcubation time on the transient expression of tPA in transfected CHO cells as determined by the quantity of tPA detected the host cells by ELISA The DNA-calcium phosphate co-precipitation was earned out with 25 or 50 μg/ml DNA, 250 mM Ca 2+ , and 0 75 mM P0 4 3" Open and closed columns represent DNA concentrations of 25 and 50 μg/ml, respectively Fig 7 is a graph depicting the solubility of calcium phosphate as a function of Ca 2+ concentration and P0 4 3 concentration in a solution containing 140 mM NaCl, 30 mM N-3-hydroxyethylpιperazιne-N'-3- ethanesulfomc acid (HEPES), pH 7 2 at 37° C Fig 8 is a graph depicting the solubility of calcium phosphate as a function of Ca 2* concentration and pH in a solution contaimng 30 mM HEPES and 0 95 mM P0 4 3 m PS04 medium at 37° C Fig 9 is a graph depicting the effects of DNA-calcium phosphate co-precipitation mixture P0 4 3 concentration and transfection mixture mcubation time on the transient expression of TNK tPA m 293 host cells transfected m suspension culture Expression level is presented as the quantity of TNK tPA detected m the suspension culture supernatant by ELISA The DNA-calcium phosphate co-precipitation was earned out with 50 μg/ml DNA, 125 mM Ca 2+ , and 0 75, 1 125 or 1 5 mM PO^ The darkly shaded column (far left) at each P0 4 concentration represents 20 hours of transfection mixture mcubation (denoted as "B") and 27 hours (total) of culture incubation post transfection The solid column (middle left) at each P0 4 3" concentration represents 20 hours of transfection mixture mcubation (denoted "B") and 52 hours (total) of culture mcubation post transfection The open column (middle nght) at each P0 4 3" concentration represents 4 hours of transfection mixture mcubation (denoted as "A") and 27 hours (total) of culture mcubation post transfection The lightly shaded column (far nght) at each P0 4 3" concentration represents 4 hours of transfection mixture mcubation (denoted as "A") and 52 hours (total) of culture mcubation post transfection Fig 10 is a graph depicting the effects of transfection mixture Ca 2+ concentration and transfection mixture incubation time on the transient expression of TNK tPA in 293 host cells transfected m suspension culture Expression level is presented as the quantity of TNK tPA detected in the suspension culture supernatant by ELISA The DNA-calcium phosphate co-precipitation was earned out with 50 μg ml DNA, 125 mM Ca 2+ , and 1 125 mM P0 4 3' A control transfection was performed by contacting a cell suspension with DNA and Ca 2+ alone (without pnor co-precipitation in the presence of PO^) The Ca 2+ concentrations indicated denote the initial Ca 2+ concentrations m the transfection mixtures The darkly shaded column (far left) at each Ca + concentration represents 20 hours of transfection mixture mcubation (denoted as "B") and 27 hours (total) of culture mcubation post transfection The solid column (middle left) at each Ca 2+ concentration represents 20 hours of transfection mixture mcubation (denoted as "B") and 52 hours (total) of culture mcubation post transfection The open column (middle nght) at each Ca 2+ concentration represents 4 hours of transfection mixture mcubation (denoted as "A") and 27 hours (total) of culture mcubation post transfection The lightly shaded column (far nght) at each Ca 2+ concentration represents 4 hours of transfection mixture mcubation (denoted as "A") and 52 hours (total) of culture mcubation post transfection Fig 1 1 is a graph depicting a time course for the cell density and the transient expression of DNase in 293 host cells transfected m suspension culture Expression level is presented as the quantity of DNase detected in the suspension culture supernatant by ELISA The time of transfection is designated as 0 hours The X's represent cell density as the packed cell volume (PCV) percentage of total culture volume and the open squares represent DNase titer m μg/ml DNA-calcium phosphate co-precipitation was performed with 20 μg ml DNA, 250 mM Ca 2+ , and 0 75 mM P0 4 3' m a total volume of 60 ml The co-precipitate was incubated for 1 -2 minutes at room temperature and then admixed to a 1 liter suspension culture at 0 19% PCV, yielding an initial Ca 2* concentration of 15 mM m the transfection mixture After 3 5 hours of mcubation at 37° C, 1 liter of fresh medium was added to the transfection mixture DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A DEFINITIONS As used herein, the term "transfection" is defined as the introduction of an extracellular nucleic acid into a host cell by any means known m the art, mcludmg calcium phosphate co-precipitation, viral transducUon, liposome fusion, microinjection, microparticle bombardment, electroporation, etc The terms "uptake of nucleic acid by a host cell", "taking up of nucleic acid by a host cell", "uptake of particles compnsing nucleic acid by a host cell", and "taking up of particles compnsing nucleic acid by a host cell" denote any process wherem an extracellular nucleic acid, with or without accompanying matenal, enters a host cell As used herein, the terms "nucleic acid-calcium phosphate co-precipitation" and "calcium phosphate co-precipitation" refer to a process wherem nucleic acid, Ca 2+ , and P0 4 3' m solution form insoluble particles, i e , a precipitate, compnsing hydroxyapatite (which has an approximate chemical formula of (Ca 5 OH(P0 4 ) 3 ) 2 , and is referred to herein as "calcium phosphate") and nucleic acid Also included within the definition is the growth of such particles by further precipitation or by aggregation and or rearrangement of such particles As used herein, the term "calcium phosphate transfection" refers to any method of transfecting a host cell wherem calcium phosphate is used to facilitate the uptake of nucleic acid by a host cell As used herein, the term "transformation" denotes introducing nucleic acid into a host cell so that the nucleic acid is rep cable, either as a chromosomal integrant or as an extrachromosomal element As used herein, the term "eukaryotic host cell lacking a cell wall" refers to any nucleated cell which has no cell wall m the cell's native state, mcludmg all vertebrate cells, such as mammalian cells, avian cells, reptilian cells, amphibian cells, and fish cells, cells of multicellular invertebrate animals, such as insect cells, crustacean cells, and mollusk cells, cells of protozoans, etc , and to any nucleated cell which has had its native cell wall removed or is in a natural or artificially induced state wherem no cell wall is present, mcludmg all plant cells that are capable of forming protoplasts or are capable of being treated to form protoplasts As used herein, the term "desired nucleic acid" refers to any desired DNA, RNA or DNA/RNA hybnd As used herein, the term "desired DNA" is defined as any polydeoxynucleotide, mcludmg, e g , double stranded DNA, single stranded DNA, double stranded DNA wherem one or both strands is (are) composed of two or more fragments, double stranded DNA wherem one or both strands has (have) an uninterrupted phosphodiester backbone, DNA containing one or more single stranded portion(s) and one or more double stranded portion(s), double stranded DNA wherein the DNA strands are fully complementary, double stranded DNA wherein the DNA strands are only partially complementary, circular, covalently closed DNA, linear DNA, covalently cross-linked DNA, cDNA, chemically synthesized DNA, semisynthetic DNA, biosynthetic DNA, naturally isolated DNA, enzyme digested DNA, sheared DNA, plasmid DNA, chromosomal DNA, labelled DNA, such as radiolabelled DNA and fluorochrome-labelled DNA, DNA containing one or more nonnaturally occurring species of nucleic acid, etc., that is selected for transfecting a host cell. As used herein, the term "desired RNA" is defined as any polyribonucleotide, including, e.g., single stranded RNA, double stranded RNA, double stranded RNA wherein one or both strands is (are) composed of two or more fragments, double stranded RNA wherein one or both strands has (have) an uninterrupted phosphodiester backbone, RNA containing one or more single stranded portion(s) and one or more double stranded portion(s), double stranded RNA wherein the RNA strands are fully complementary, double stranded RNA wherein the RNA strands are only partially complementary, covalently cross-linked RNA, enzyme digested RNA, sheared RNA, mRNA, hnRNA, tRNA, including both charged and uncharged tRNA, rRNA, all forms of viral genomic RNA, chemically synthesized RNA, semisynthetic RNA, biosynthetic RNA, naturally isolated RNA, labelled RNA, such as radiolabelled RNA and fluorochrome-labelled RNA, RNA containing one or more nonnaturally occurring species of nucleic acid, etc., that is selected for transfecting a host cell. As used herein, the terms "desired DNA/RNA hybrid" and "desired hybrid DNA/RNA" are defined as any hybrid nucleic acid comprising one strand of DNA and one strand of RNA wherein the DNA strand and the RNA strand form a species that is at least partially double stranded, including hybrids wherein the DNA strand is fully complementary or only partially complementary to the RNA strand, hybrids wherein the DNA strand and/or the RNA strand has (have) an uninterrupted phosphodiester backbone, hybrids wherein the DNA strand and/or the RNA strand is composed of two or more fragments, hybrids containing one or more single stranded portion(s) and one or more double stranded portion(s), hybrids created by reverse transcription of RNA, hybrids created by transcription of DNA, hybrids created by annealing of complementary or partially complementary DNA and RNA, covalently cross-linked hybrids, chemically synthesized hybrids, semisynthetic hybrids, biosynthetic hybrids, naturally isolated hybrids, labelled hybrids, such as radiolabelled hybrids and fluorochrome-labelled hybrids, hybrids containing one or more nonnaturally occuring species of nucleic acid, etc. B. GENERAL METHODS In general, the invention provides for methods of calcium phosphate transfection in suspension culture. The methods provided herein are particularly useful for transfection in large scale suspension cultures, e.g., suspension cultures that are least 0.5 liters (L) in volume. The invention also provides for improved methods of calcium phosphate transfection in adherent cell cultures. The methods of the invention exploit the heretofore unknown properties of nucleic acid-calcium phosphate co-precipitation and calcium phosphate facilitated nucleic acid transfection of host cells. In general, calcium phosphate transfection proceeds as follows. Nucleic acid associates strongly with the calcium phosphate particles formed in a calcium phosphate precipitate. In the presence of host cells, calcium phosphate particles carrying nucleic acid contact the host cell surface, and the nucleic acid enters the host cell. The present inventors discovered that there is a correlation between the host cell's ability to take up nucleic acid and the size of the nucleic acid-calcium phosphate particles, and that there is an optimum particle size that maximizes the host cell's ability to take up nucleic acid Although the mvention is not limited to any particular mechanism of action, it is believed that the limitation of particle size can increase transfection efficiency by increasing the number of particles that come into contact with a host cell The limitation of particle growth after particles reach a particular size reduces the decline in particle number caused by aggregation or rearrangement of smaller particles The limitation of particle size can also mcrease transfection efficiency by optimizing the particle surface area available for association with nucleic acid As shown herein, the present inventors have determined that the optimum particle size is any size up to about 300 nm m length, wherem "length" is defined as the diameter at the widest part of the particle as measured by laser light scattering according to the method of Weiss and Frock, "Rapid Analysis of Particle Size Distnbution by Laser Light Scattering", Powder Technology, 14 287 (1976) Thus, the methods of the mvention are designed to maximize the host cell's uptake of nucleic acid by exposing the host cell to particles that have an average length of up to about 300 nm It is not possible to maintain calcium phosphate particles at a constant size because the particles are not stable in any liquid mixture The particles either grow by precipitation, aggregation and rearrangement or dissolve, dependmg on the solubility of calcium phosphate in the mixture Accordmgly, the mvention provides methods wherem the particles are grown to an optimal size m a nucleic acid-calcium phosphate co-precipitation step, the particles are diluted to lower the particle growth rate while maintaining particle insolubility, and the optimally sized particles are contacted with the host cell The mvention also provides methods wherem the particles are grown larger than the optimal size, reduced back down to the optimal size and contacted with the host cell A host cell can be exposed to the optimally-sized particles in at least four ways (1) formmg the optimally-sized particles in a DNA-calcium phosphate co-precipitate, and then diluting the co-precipitate and contacting it with the host cell in a single step by admixmg the co-precipitate to a host cell culture, (2) formmg the optimally-sized particles m a DNA-calcium phosphate co-precipitate, diluting the co-precipitate, and then admixmg the diluted co-precipitate to a host cell culture, (3) forming the optimally-sized particles in a host cell culture, and then diluting the host cell culture, and (4) formmg particles that are larger than the optimal size, shrinking the particles back down to the optimal size and contacting the particles with the host cells I Simultaneous Dilution of Co-Precipitate and Exposure to Host Cell a Host Cell Preparation Any eukaryotic host cell lacking a cell wall can be used in the methods of the mvention Preferred for use herein are mammalian cells Examples of useful mammalian host cell lmes include monkey kidney CV1 line transformed by SV40 (COS-7, ATCC CRL 1651), human embryonic kidney line (293 or 293 cells subcloned for growth m suspension culture, Graham et al , J Gen Virol , 36 59 (1977)), baby hamster kidney cells (BHK, ATCC CCL 10), Chinese hamster ovary cells/-DHFR (CHO, Urlaub and Chasm, Proc Natl Acad Set USA, 77 4216 ( 1980)), mouse sertoh cells (TM4, Mather, Btol Reprod , 23: 243-251 ( 1980)), monkey kidney cells (CV1 ATCC CCL 70), African green monkey kidney cells (VERO-76, ATCC CRL-1587), human cervical carcinoma cells (HELA, ATCC CCL 2), canine kidney cells (MDCK, ATCC CCL 34), buffalo rat liver cells (BRL 3A, ATCC CRL 1442), human lung cells (W138, ATCC CCL 75), human liver cells (Hep G2, HB 8065), mouse mammary tumor (MMT 060562, ATCC CCL51); TRI cells (Mather et al. Annals N.Y. Acad. ScL, 383: 44-68 (1982)); MRC 5 cells; FS4 cells; and a human hepatoma cell line (Hep G2). The mammalian host cell of choice can be cultured by any method known in art, such as, e.g., growing the cells as a monolayer with Dulbecco modified Eagle medium (DMEM) supplemented with 10% calf serum in an incubator at 35° C under a 5% CO- atmosphere. Other procedures can be used for particular cell types For example, Drosophila cell lines can be grown as described by Di Nocera and Dawid, Proc. Nail. Acad. Sci. USA, 80: 7095-7098 (1983) and fish cell lines can be grown as described by Araki et al.. Bull. Natl. Res. Inst. Aquaculture, 20: 1-9 (1991). Alternatively, a suspension cell culture can be used. Cells in suspension can be grown in spinner flasks, ranging in volume from 100 milliliters (ml) to 10 liters (L) or in bioreactors ranging in volume from 0.5 L to 10,000 L. Cells in a suspension culture must be kept in an exponential growth phase which can be achieved by several methods known in the art, the most common of which is subcultivation with fresh medium every 3 to 6 days. Standard techniques, methods and equipment are reviewed in Lubiniecki, ed, Large Scale Mammalian Cell Culture Technology, Marcel Dekker, New York and Basle, 1990. In the case of plant cell hosts, the plant cell protoplast cultures suitable for use herein can be prepared according to the method of Lichtenstein and Draper, "Genetic Engineering of Plants", in DNA Cloning Volume III: A Practical Approach, Glover, ed, IRL Press (1985), pp.67-119. b. DNA Preparation Any desired DNA for use in the methods of the invention can be prepared by a variety of methods known in the art. These methods include, but are not limited to, chemical synthesis by any of the methods described in Engels et al, Angew. Chem. Int. Ed. Engl, 28: 716-734 (1989), the entire disclosure of which is incorporated herein by reference, such as the triester, phosphite, phosphoramidite and H-phosphonate methods. Alternatively, the desired DNA sequences can be obtained from existing clones or, if none are available, by screening DNA libraries and constructing the desired DNA sequences from the library clones. Suitable quantities of DNA template for use herein can be produced by amplifying the DNA in well known cloning vectors and hosts, such as plasmid vectors carrying the pBR322 origin of replication for autonomous replication in most Gram-negative bacterial hosts, plasmid vectors carrying the pC194 (Ehrlich, Proc. Natl Acad. Sci. USA, 75: 1433-1436 (1978)) origin of replication for autonomous replication in Bacillus and some other Gram-positive bacterial hosts, or 2 -micron circle (2μ plasmid) vectors carrying an origin of replication for autonomous replication in most yeast hosts. Alternatively, the DNA template can be amplified by polymerase chain reaction (PCR) as described by Saiki e. tf/., &»e«c- \ 230: 1350 (1985), Mullis ef al. Cold Spring Harbor Symp. Quant. Biol, 51: 263 (1986), Mullis and Faloona, Methods Enzymol, 155: 335 (1987), and Saiki et al, Science, 239: 487 (1988). c. RNA Preparation Any desired RNA for use in the methods of the invention can be prepared by a variety of methods known in the art. These methods include, but are not limited to, chemical synthesis of RNA, and in vitro translation of a DNA template as descnbed generally in Current Protocols in Molecular Biology, Wiley Interscience, New York (1990) Alternatively, the desired RNA can be isolated from total cellular RNA extracted from a host cell culture Total cellular RNA can be isolated from the host cell culture by any method known m the art such as, m the case of RNA produced in mammalian host cells, the methods descnbed by Favaloro et al , Methods Enzymol, 65 718 (1980), Stallcup and Washington, J Biol Chem , 258 2802 (1983), Bimboim, Nucleic Acids Res , 16 1487 (1988), Gilsin et al , Biochemistry, 13 2633 (1974), Ullnch et al , Science, 196 1313 (1977), Strohman et al , Cell, 10 265 (1977), and MacDonald et al , Methods Enzymol , 152: 219 (1987) If the desired RNA is a polyadenylated mRNA fraction of total cellular RNA, the polyadenylated mRNA can be separated from the bulk of cellular RNA by affinity chromatography on oligodeoxythymidylate (ohgo(dT))-cellulose columns using any method known m the art, such as the method of Edmonds et al , Proc Natl Acad Sci , 68 1336 ( 1971 ) or the method of Aviv and Leder, Proc Natl Acad Sci , 69 1408 ( 1972) If the size of the desired mRNA is known, the mRNA preparation can be further purified for mRNA molecules of the particular size by agarose gel electrophoresis of RNA in the presence of methvlmercuπc hydroxide as descnbed m Lemischka et al , J Mol Btol , 151 101 (1981) or fractionation of RNA by sucrose density gradient centnfugation in the presence of methylmercunc hydroxide as descπbed bv Schwemfest et al , Proc Natl Acad Sci , 79 4997 (1982) In addition, the desired RNA can be obtained from the recombinant or non-recombinant genome of an RNA virus, mcludmg single stranded RNA viruses, such as retroviruses, tobacco mosaic viruses, influenza viruses, Newcastle disease virus, and double stranded RNA viruses such as rotavmises and πce dwarf virus The desired RNA can be isolated by growing up the chosen RNA virus m a suitable host cell culture, harvesting the viral particles and then extracting the desired RNA from the viral particles For example, the genomic RNA of Moloney's munne leukemia virus can be obtained according to the method of Schwartzberg et al , Cell, 37 1043 (1984) d DNA/RNA Hvbnd Preparation The DNA/RNA hybnds suitable for use m the methods of the mvention can be prepared by any method known in the art In one embodiment, the DNA strand or DNA fragments is (are) produced as descnbed m Section 1(b) above, the RNA strand or fragments is (are) produced as descπbed in Section 1(c) above, and the DNA and RNA strands or fragments are admixed together and allowed to anneal In another embodiment, the DNA/RNA hybnd can be produced by obtaining the desired DNA strand as descnbed above, using the DNA strand as a template to dnve synthesis of the complementary RNA strand by a DNA-directed RNA polymerase, and harvesting the DNA/RNA hybnd upon completion of the transcnption reaction Alternatively, the DNA/RNA hybnd can be obtained by obtaining the desired RNA strand as descnbed above, using the RNA strand as a template to drive synthesis of the complementary DNA strand by a RNA-directed DNA polymerase, and harvesting the DNA/RNA hybnd upon completion of the reverse transcnption reaction e Procedure for calcium phosphate transfection The invention encompasses any method for introducing a desired nucleic acid mto a eukaryotic host cell wherem the desired nucleic acid, Ca 2+ , and P0 4 3' are admixed to form a precipitation mixture, the precipitation mixture is incubated to form particles compnsmg calcium phosphate and the desired nucleic acid and the particles are allowed to grow to an average length of up to about 300 nm, the precipitation mixture is simultaneously diluted and admixed to a eukaryotic host cell lackmg a cell wall to form a transfection mixture wherem the particles are capable of growth at a rate that is substantially lower than the rate at which the particles grew m the precipitation mixture, and the transfection mixture is incubated to allow the host cell to take up the particles to form a transfected cell 1 Formation of the Precipitation Mixture Ca 2+ , P0 4 3 and the desired nucleic acid can be admixed m any order to form a precipitation mixture wherem the nucleic acid co-precipitates with calcium phosphate In one embodiment, the number of particles compnsmg nucleic acid and calcium phosphate formed m the precipitation mixture are maximized by admixing the nucleic acid to the precipitation mixture before or simultaneously with the admixture of Ca + and P0 4 3" The nucleic acid can be suspended in a buffer lackmg both Ca 2+ and P0 4 3 and then Ca + and P0 4 3 can be consecutively or simultaneously admixed to the nucleic acid suspension Alternatively, the nucleic acid can be suspended in a buffer containmg Ca + or P0 4 3" and then the appropnate counteπon can be admixed to the nucleic acid suspension to initiate co-precipitation The concentration of reactants and the reaction conditions m the precipitation mixture are selected to produce particles compnsmg nucleic acid and calcium phosphate that have an average length of up to about 300 nm In a preferred embodiment- the reactant concentrations and reaction conditions are selected to produce particles that have an average length of about 100 nm or less The primary factors that determine particle formation are the kinetics of particle growth the mixture and the reaction time The rate of particle growth dictates the reaction time needed to attain the desired particle size The rate of particle growth is dependent on the concentration of nucleic acid and the solubility of calcium phosphate m the mixture, and the calcium phosphate solubility, in turn, is dependent on the Ca + concentration, P0 4 concentration, pH, and temperature of the precipitation mixture The Ca 2+ concentration, P0 4 3' concentration, pH, and temperature of the precipitation mixture are selected to provide a calcium phosphate solubility well below the actual Ca 2+ concentration and P0 4 3 concentration m the iruxture, thus providing a supersaturation of Ca + and P0 4 3" ions that dnves co-precipitation of calcium phosphate and nucleic acid Nucleic acid concentration also affects the kinetics of particle growth m the supersaturated mixture because nucleic acids adhere to the surface of the particles, thereby slowing particle growth In the precipitation mixture, Ca 2+ can be present at an initial concentration of about 125 mM to about 375 mM, and preferably about 250 mM to about 375 mM, and P0 4 3' can be present at an initial concentration of about 0 5 mM to about 1 125 mM and preferably about 0 75 mM At a given P0 4 3" concentration, a higher Ca 2+ concentration can result m formation of particles with greater speed and frequency Accordmgly, the optimum nucleic acid concentration can vary with the Ca 2+ concentration in the precipitation mixture In DNA transfection embodiments using initial Ca 2+ concentrations of about 125 mM, 250 mM and 375 mM with an initial P0 4 3 concentration of about 0 75 mM m the precipitation mixture, the initial DNA concentrations m the precipitation mixture can be up to about 25 μg/ml, 50 μg/ml, and 75 μg/ml, respectively Alternatively, at a given Ca 2+ concentration higher P0 4 3" concentrations can be used to produce the desired speed and frequency of calcium phosphate crystal formation Accordingly, the optimum nucleic acid concentration can vary with the PO,, 3" concentration in the precipitation mixture In DNA transfection embodiments using an initial P0 4 3 concentration of about 1 125 mM, the initial DNA concentration can be up to about 50 μg/ml Due to volumetric constraints m large scale suspension culture transfection, it is desirable to increase the Ca 2+ concentration m the suspension culture pπor to inoculation with the precipitation mixture Under these circumstances, it is advantageous to reduce the Ca + concentration m the precipitation mixture to compensate for the increased Ca concentration m the suspension culture In turn, the initial P0 4 concentration m the precipitation mixture is increased to maintain the desired rate of calcium phosphate particle formation In one suspension culture embodiment, P0 4 and Ca + are present m the precipitation mixture at initial concentrations of 1 125 mM and 125 mM, respectively The pH of the precipitation mixture can be about 6 8 to about 7 6, and is preferably about 7 05 The temperature of the precipitation mixture can be about 0° C to about 37° C, preferably about 20° C to about 37° C, and more preferably about 32° C to about 37° C However, any precipitation mixture mcubation temperature, including any temperature outside of the foregomg temperature ranges, that combines with the other reaction parameters to produce the desired rate of particle formation is contemplated for use m the methods of the invention In one embodiment for transfection in suspension culture, the precipitation mixture is mcubated at a temperature of about 0° C to about 50° C Any pH buffer that is effective at a pH range encompassing the desired pH for the precipitation mixture can be used to suspend the reactants m the precipitation mixture Buffers that are suitable for use herein include appropnate concentrations of N-3-hydroxyethylpιperazme-N-3-ethanesulfomc acid (HEPES)-buffered saline, such as 25 mM HEPES and 140 mM NaCl, and appropnate concentrations of N,N-bιs(3-hydroxyethyl)-3- ammoethanesulfonic acid (BES)-buffered saline, such as 25 mM BES and 140 mM NaCl The precipitation mixture is mcubated for any penod of time sufficient to allow particles compnsmg calcium phosphate and nucleic acid to grow to an average length of up to about 300 nm, and preferably an average length of about 100 nm or less Under the reaction conditions descnbed above, the particles formed in the precipitation mixture can have an average length of about 300 nm or less after an mcubation penod of up to about 60 seconds Accordmgly, the precipitation mixture can be mcubated for up to about 60 seconds In a preferred embodiment, the precipitation mixture is mcubated for a penod of about 30 seconds or less However, the mvention also encompasses the use of other mcubation penods to obtain the desired particle size under vaπous reaction conditions in the precipitation mixture In some embodiments for transfecting cells m suspension, the precipitation mixture can be mcubated for a penod of up to about one hour, but is preferably mcubated for a penod of up to about 2 minutes, and more preferably for a penod of up to about 1 minute, and still more preferably for a period of up to about 30 seconds 2 Formation of the Transfection Mixture After particles compnsmg calcium phosphate and the desired nucleic acid have grown to an average length of up to about 300 nm (or less) m the precipitation mixture, the precipitation mixture is simultaneously diluted and admixed to a eukaryotic host cell lackmg a cell wall to form a transfection mixture The eukaryotic cell is obtained m the form of an adherent cell culture or a suspension cell culture as descπbed in section 1(a) above As provided herein, the precipitation mixture is diluted by admixture to the host cell culture such that the growth rate of the particles m the transfection mixture is substantially lowered, compared to the growth rate of the particles in the precipitation mixture, without allowing resolvation of the particles, thereby maximizing the exposure of host cells to optimally sized particles In a preferred embodiment using a suspension cell culture, the precipitation and dilution steps are accomplished m an automated system wherem nucleic acid, Ca 2+ , and P0 4 3 are fed mto an intake pipe that empties mto the culture vessel The nucleic acid, Ca 2+ , and P0 4 3 can be fed mto the intake pipe m any convenient order Preferably, the nucleic acid is fed mto the intake pipe upstream of the initial pomt of calcium phosphate precipitate formation Alternatively, the nucleic acid, Ca 2+ , and P0 4 3 are fed mto the intake pipe at approximately the same pomt In one embodiment, a solution containmg the nucleic acid and one of the two ions and a solution containmg the countenon are fed through separate tube lmes that merge mto the intake pipe The flow rate through the intake pipe and the intake pipe length can be regulated to achieve the desired mcubation period for nucleic acid-calcium phosphate co-precipitation within the intake pipe Preferably, the suspension culture is agitated to maximize the contact between host cells and optimally sized particles of calcium phosphate and nucleic acid The methods of the mvention can be used to transfect cells in a suspension culture of any size Preferably, the methods of the mvention are used for transfection of suspension cultures compnsmg a total volume of at least about 1 liter, and more preferably compnsmg a total volume of at least about 20 liters The desired cell density for transfection m suspension culture can be achieved, e g , by growing a defined volume of the seed culture to the particular cell density Alternatively, cells from the seed culture are recovered by filtration and/or centnfugation and resuspended at the desired density In another embodiment, the desired cell density is achieved by dilution of the seed culture The cell density for transfection m suspension culture can be about 02% to about 5% packed cell volume (PCV) However, the mvention also encompasses the use of higher or lower cell densities that provide acceptable levels of transfection m suspension culture For example, the mvention can be practiced by concentrating cells from a bioreactor to obtain a high density cell slurry, and then admixmg the precipitation mixture to the cell slurry In one embodiment, a cell density of greater than about 10 s cells/ml is used In another embodiment, a cell density of about 10 β cells ml to about 10 9 cells/ml is used A concentrated slurry can be obtained by pumping cell suspension from a bioreactor mto a semi-continuous aseptic centrif ge, such as the Hereaus Sepatech Contifuge™ 17RS (1994 Hereaus Instruments Catalog No 75003571, Hereaus Instruments Gmbh, D63405, Hanau, Germany) and centnfugmg the cell suspension at about 500 x g to about 6,000 x g, and preferably about 5,300 x g, to entrap the cells in an aseptic rotor bowl Depending on the cell density of the bioreactor cell culture, up to about 100 liters of suspension culture can be harvested m the centrifuge rotor bowl Subsequently, the high density cell slurry is removed from the bowl and admixed to the DNA-calcium phosphate co-precipitate to form the transfection mixture In one embodiment, the high density cell slurry is admixed to the DNA-calcium phosphate co-precipitate in an intake pipe to form the transfection mixture inline pnor to entering the bioreactor Alternatively, the DNA-calcium phosphate co-precipitate and high density cell slurry can be separately introduced mto the bioreactor through feeding and inoculation ports, respectiveh Also provided herem are embodiments m which the cell concentration m the high density cell slurry is adjusted, e g , by addition of fresh growth medium, to reach the optimal concentration for transfection pnor to the formation of the transfection mixture It will be appreciated that the useful cell concentrations for transfection of a particular host cell can easily be determined with routine testing In another embodiment using a suspension cell culture, the Ca * concentration m the suspension culture is increased prior to inoculation with the precipitation mixture as descπbed above m Section (e)(1) In a preferred embodiment, the Ca 2+ concentration m the suspension culture is raised to about 7 5 mM pnor to inoculation with the precipitation mixture The mvention encompasses the use of any method for substantially lowering the particle growth rate to restnct particle growth after the optimum particle size is reached, mcludmg decreasmg Ca 2+ concentration, decreasmg P0 4 concentration, decreasmg pH, increasing nucleic acid concentration, increasing protem concentration, decreasmg temperature, or any combmation thereof In one embodiment, the precipitation mixture is diluted by admixture to the host cell culture to form a transfection mixture with an initial Ca 2* concentration of about 9 mM to about 17 mM In yet another embodiment, the transfection mixture has an initial Ca 2* concentration that is at least ten-fold lower than the initial Ca 2+ concentration of the precipitation mixture In a preferred embodiment, the precipitation mixture has an initial Ca 2+ concentration of about 250 mM to about 375 mM and the transfection mixture has an initial Ca 2+ concentration of about 12 mM Preferably, the particle growth rate is substantially reduced by the presence of serum or serum protem, such as bovme serum albumin, the transfection mixture Protein, like nucleic acid, associates strongly with the calcium phosphate particle surface and thereby impedes particle growth In one embodiment, the transfection mixture contains about 2% to about 10% serum, such as fetal calf serum In another embodiment, the transfection mixture contains about 0 2 grams per liter (g/L) to about 4 g/L serum albumm, such as bovme serum albumm The initial P0 4 3' concentration of the transfection mixture used m the present methods can be conveniently provided by the P0 4 3" present in the cell culture medium Although other concentrations are also appropnate, the P0 4 3 concentration of about 1 mM m serum-supplemented cell culture media is sufficient for use herem The pH and temperature of the transfection mixture are maintained at physiological levels tolerated by the host cells In the case of mammalian host cells, it is desirable to maintain the pH in the range of about 60 to about 8 0, and preferably about 7 2 to about 7 5, and the temperature m the range of about 15°C to about 39° C, and preferably about 32° C to about 37° C. Similarly, the transfection mixture is mcubated for a period of time that is easily optimized for the particular host cell In the case of transfection a suspension cell culture, it is possible to precisely regulate the pH, Ca 2* concentration, PO,, 3* concentration and temperature such that the solubility of the particles compnsmg calcium phosphate and the desired nucleic acid is as low as possible without permitting resolvation of the particles In a preferred embodiment- the transfection mixture is maintained at a pH of about 7 2 and a temperature of about 37° C and the Ca 2+ concentration and the PO^ concentration therein are maintained such that the P0 4 3 and Ca 2* concentrations define a pomt that is above the calcium phosphate solubility curve shown m Fig 7 In another preferred embodiment, the transfection mixture is maintained with a P0 4 3 concentration of about 1 mM and a temperature of about 37° C and the Ca 2 * concentration and pH are maintained such that the Ca * concentration and pH define a pomt above the calcium phosphate solubility curve shown m Fig 8 Calcium phosphate precipitate is toxic to some host cells Accordingly, it can be advantageous to dissolve the precipitate after the desired mcubation penod for transfection The calcium phosphate precipitate m the transfection mixture can be dissolved, e g , by lowering the pH and/or lowering the Ca 2 * concentration m the transfection mixture The Ca * concentration can be conveniently lowered by adding fresh culture medium to the transfection mixture In one suspension culture embodiment, the transfection mixture is mcubated for about 1 minute to about 24 hours, and preferably about 0 5 hours to about 12 hours, and more preferably about 3 5 hours to about 5 hours, and then diluted with about 1 volume to about 500 volumes of cell culture medium and mcubated for about 1 day to about 14 days For some host cells, an improved rate of transfection is obtained by shocking the transfection mixture with glycerol or dimethylsulfoxide (DMSO) at the end of the transfection mcubation penod Typically, the transfection mixture is exposed to glycerol at a concentration of about 15% volume volume for about 30 seconds to about 3 minutes, dependmg on the particular host cell, and then the cells are mcubated m fresh medium for about 1 to 6 days Alternatively, following transfection the host cells can be cultured in fresh medium for the desired time penod without a glycerol shock II Dilution of Co-precipitate Followed bv Exposure to the Host Cell The invention also encompasses any method for introducing a desired nucleic acid mto a eukaryotic host cell wherem the desired nucleic acid, Ca 2 *, and PO^ are admixed to form a precipitation mixture, the precipitation mixture is mcubated to form particles compnsmg calcium phosphate and the desired nucleic acid and the particles are allowed to grow to an average length of up to about 300 nm, the precipitation mixture is diluted to form a ώluted precipitation mixture, the diluted precipitation mixture is admixed to a eukaryotic host cell lacking a cell wall to form a transfection mixture wherem the particles are capable of growth at a rate that is substantially lower than the rate at which the particles grew m the precipitation mixture, and the transfection mixture is mcubated to allow the host cell to take up the particles to form a transfected cell a Formation of the Precipitation Mixture The precipitation mixture is obtained and mcubated as descπbed m Section 1(e)(1) above After the desired nucleic acid-calcium phosphate co-precipitate is formed, the precipitation mixture can be diluted by any convenient means, e g , by adding an appropnate buffer or by adding the cell culture medium to be used m transfection Buffers and media suitable for use herem are descnbed m Sections 1(a) and 1(e)(1) above The diluent is added to the precipitation mixture m an amount sufficient to reduce the rate of nucleic acid-calcium phosphate particle growth but not allow resolvation of such particles m the resulting diluted precipitation mixture Until it is admixed to a host cell to form a transfection mixture, the diluted precipitation mixture is maintained under conditions that permit continued but slow growth of the nucleic acid-calcium phosphate particles Suitable conditions for obtaining a slow particle growth rate are set forth m the descnption of the transfection mixture m Section 1(e)(2) above b Formation of Transfection Mixture As provided herem, the diluted precipitation mixture is admixed to a eukaryotic host cell lacking a cell wall to form a transfection mixture wherem the DNA-calcium phosphate particles will grow at a substantially lower rate than the particle growth rate m the precipitation mixture The eukaryotic cell is obtained in the form of an adherent cell culture or a suspension cell culture as descnbed m Section 1(a) above, and the diluted precipitation mixture can be admixed to the cell culture to form a transfection mixture as descπbed m Section 1(e)(2) above The dilution of the particles m the diluted precipitation mixture and the dilution of the particles in the transfection mixture are chosen such that the overall dilution substantially lowers the particle growth rate without permitting the particles to dissolve In a preferred embodiment, the overall dilution provides an initial Ca 2* concentration m the transfection mixture that is at least ten-fold lower than the initial Ca 2* concentration in the precipitation mixture In another preferred embodiment, most of the overall dilution occurs m the formation of the diluted precipitation mixture in order to slow down particle growth as soon as possible after the optimal particle size is attained Nevertheless, it will be appreciated that the mvention also encompasses methods wherem the dilution that occurs m formation of the diluted precipitation mixture is small m companson to the overall dilution in the transfection mixture In such embodiments, it is preferable that the diluted precipitation mixture be quickly admixed to the host cell culture m order to attain a substantial decrease particle growth rate before the average particle size exceeds the optimal range Alternatively, the percentage of the overall dilution that occurs m the formation of the diluted precipitation mixture and the percentage of the overall dilution that occurs m the formation of the transfection mixture can be vaned accordmg to the length of time between the two steps A short time mterval would permit the use of a smaller dilution m the diluted precipitation mixture whereas a longer time mterval would necessitate the use of a larger dilution m the diluted precipitation mixture m order to prevent undue loss of transfection activity Preferably, the diluted precipitation mixture is immediately admixed to host cells m order to maximize the host cells' exposure to optimally sized nucleic acid-calcium phosphate particles However, the mvention also encompasses embodiments wherem the diluted precipitation mixture is maintained for any penod of time before admixture to the host cells provided that the diluted precipitation mixture retains some ability to transfect the host cells at the time the transfection mixture is formed In a preferred embodiment using a suspension cell culture, the precipitation and dilution steps are accomplished in an automated system wherem nucleic acid, Ca 2* , and P0 4 3" feed mto an intake pipe that allows nucleic acid-calcium phosphate co-precipitation to occur, diluent feeds mto the precipitation mixture through another intake pipe at some pomt downstream of the nucleic acid, Ca 2 *, and P0 4 3' intake, and thereafter the diluted precipitation mixture empties mto the culture vessel The nucleic acid, Ca 2 *, and P0 4 3" can be fed mto the intake pipe in any convenient order as descnbed m Section 1(e)(2) above The flow rate through the intake pipe that carries the precipitation mixture and the downstream positioning of the diluent intake pipe and the culture vessel entry port can be adjusted to achieve the desired mcubation penod for the precipitation mixture and the desired delay between dilution of the precipitation mixture and admixture to the host cells in the culture vessel Preferably, the suspension culture is agitated to maximize the contact between host cells and optimally sized particles of calcium phosphate and nucleic acid After it is formed, the transfection mixture can be mcubated under the conditions descnbed in Section 1(e)(2) above HI Formation of Co-Precipitate Host Cell Culture The invention also encompasses any method for introducing a desired nucleic acid mto a eukaryotic host cell wherem Ca 2* , P0 4 3 , nucleic acid, and a eukaryotic host cell lackmg a cell wall are admixed to form a precipitation mixture, the precipitation mixture is mcubated to form particles compnsmg calcium phosphate and the desired nucleic acid and the particles are allowed to grow to an average length of up to about 300 nm, the precipitation mixture is diluted to form a transfection mixture wherem the particles are capable of growth at a rate that is substantially lower than the rate at which the particles grew m the precipitation mixture and the transfection mixture is mcubated to allow the host cell to take up the particles to form a transfected cell a Formation of the Precipitation Mixture A suitable host cell culture can be obtained as descnbed m Section 1(a) above The growth medium is removed from the cells, and the cells are exposed to appropnate concentrations of nucleic acid, Ca 2 *, and P0 4 , descπbed m Section 1(e)(1) above, to form a precipitation mixture It will be appreciated that the order of admixing nucleic acid, Ca *, and P0 4 " is not important for practicmg the mvention The cells can be contacted with or suspended a mixture containmg any of the nucleic acid, Ca 2* , and P0 4 3 components or combmation thereof and then admixed to any missing component or components needed to complete the precipitation mixture Alternatively, the cells can be admixed to all of the nucleic acid, Ca 2* , and P0 4 3" components at once In a preferred embodiment, the precipitation mixture is formed by contacting the host cells with an appropriate serum-free growth medium which compnses the desired concentrations of nucleic acid, Ca * , and P0 4 " A medium containmg serum or other proteins is undesirable for use m the precipitation mixture because proteins substantially reduce the growth of the nucleic acid-calcium phosphate co-precipitate The precipitation mixture reaction conditions and mcubation penod are selected to allow formation of optimally sized particles compnsmg calcium phosphate and nucleic acid as descnbed m Section 1(e)(1) above b Formation of the Transfection Mixture After nucleic acid-calcium phosphate particles of the desired size are formed, the precipitation mixture is diluted to form a transfection mixture wherem the particles will grow at a substantially lower rate than the particle growth rate m the precipitation mixture In one embodiment, the precipitation mixture is diluted by adding the appropnate serum-supplemented growth medium for the host cells The resulting transfection mixture is mcubated under conditions that allow the host cell to take up the nucleic acid-calcium phosphate particles to form a transfected cell Such procedures are descπbed in Section 1(e)(2) above IV Shrinking an Overgrown Co-Precipitate Followed bv Exposure to the Host Cell The invention further encompasses any method for introducing a desired nucleic acid mto a eukaryotic host cell wherem Ca 2 *, P0 4 3" and nucleic acid are admixed to form a precipitation mixture, the precipitation mixture is mcubated to form particles compnsmg calcium phosphate and the desired nucleic acid and the particles are allowed to grow to an average length that is greater than about 300 nm, the particles are decreased in size to form optimally-sized particles with an average length of about 300 nm or less by mcubatmg the precipitation mixture under conditions wherem the particles are capable of resolvation, the optimally-sized particles are admixed to a eukaryotic host cell lackmg a cell wall to form a transfection mixture, and the transfection mixture is mcubated to allow the host cell to take up the particles to form a transfected cell a Formation of the Overgrown Precipitate An overgrown nucleic acid-calcium phosphate co-precipitate can be obtained by creating a precipitation mixture as descnbed m Section 1(e)(1) above and mcubatmg the precipitation mixture under conditions that allow particles compnsmg calcium phosphate and nucleic acid to grow to an average length that is greater than about 300 nm Under the reaction conditions descnbed m Section 1(e)(1) above, the particles formed m the precipitation mixture have an average length of about 300 nm or less after an mcubation penod of about 60 seconds Accordingly, an overgrown precipitate can be formed by mcubatmg the precipitation mixture for at least about 60 seconds b Shrinking the Overgrown Precipitate After the overgrown precipitate is formed m the precipitation mixture, the conditions m the precipitation mixture are altered to cause reduction of particle bulk m the precipitate Smce particle growth or shrinkage is dependent upon the solubility of calcium phosphate in the precipitation mixture, particle shrinkage can be achieved by increasing the solubility of calcium phosphate m the precipitation mixture As descnbed above, calcium phosphate solubility is determined by Ca 2+ concentration, P0 4 3" concentration, pH and temperature Calcium phosphate solubility can be increased by lowermg the Ca concentration, lowering the P0 4 " concentration, lowermg the pH and/or lowermg the temperature of the precipitation mixture In a preferred embodiment, the precipitation mixture is diluted with an appropnate buffer to lower the Ca * and P0 4 concentrations In another embodiment, the dilution is performed by centnfuging the precipitation mixture and resuspending the pellet m an appropnate buffer to create an undersaturated solution of Ca 2* and P0 4 3' In yet another preferred embodiment, the pH of the precipitation mixture is lowered to create an undersaturated solution ofCa^ and PO^ The precipitation mixture is mcubated under conditions permitting shrinkage of the particles until the particles reach the optimal average length of about 300 nm or less, and preferably about 100 nm or less The incubation penod necessary to form optimally-sized particles vanes accordmg to the size of the overgrown particles immediately pnor to shrinkage and the speed of resolvation m the precipitation mixture Thus, the incubation time is selected based on the amount of particle size reduction needed and the rate of particle shrinkage set by the Ca 2+ and P0 4 3" undersaturation the precipitation mixture c. Formation of Transfection Mixture After the nucleic acid-calcium phosphate particles in the precipitation mixture are reduced to the optimal size, the conditions in the precipitation mixture are changed again to permit slow growth of the optimally-sized particles. In one embodiment, the precipitation mixture is admixed to a eukaryotic host cell lacking a cell wall to form a transfection mixture with an adequate Ca 2* concentration, P0 4 3" concentration, pH and temperature to permit the particles to grow at a slow rate as described in Section 1(e)(2) above. In another embodiment, the Ca * concentration, P0 4 ' concentration, pH and temperature of the precipitation mixture, or any one or combination of these parameters, is (are) increased to attain a Ca * and P0 4 3' supersaturation level that permits the particles to grow at a slow rate and the precipitation mixture is admixed to a eukaryotic host cell lacking a cell wall to form a transfection mixture. Lastly, the transfection mixture is incubated as described in Section 1(e)(2) above. V. Other Embodiments Involving Shrinkage of an Overgrown Co-Precipitate In addition to methods for shrinking an overgrown precipitate prior to contacting the precipitate and the host cell, the invention also encompasses methods wherein the overgrown nucleic acid-calcium phosphate particles are simultaneously reduced in size and exposed to the host cell. In these embodiments, the precipitation mixture is simultaneously diluted and admixed to a host cell culture to form a transfection mixture wherein the particles are capable of decreasing in size and the transfection mixture is incubated under conditions wherein the particles are reduced until they reach an average length of about 300 nm or less, and preferably about 100 nm or less. The rate of particle resolvation (determined by the Ca * concentration, P0 4 3" concentration, pH, and temperature of the transfection mixture) and incubation time are selected to provide the desired amount of particle reduction as described in Section IV(b) above. Next, the conditions in the transfection mixture are changed to permit slow growth of the particles and the transfection mixture is incubated to allow the host cell to take up the particles to form a transformed host cell as described in Section I V(c) above. Alternatively, an overgrown precipitate can be simultaneously diluted and admixed to a host cell culture to form a transfection mixture wherein the particles slowly decrease in size until they completely dissolve, which allows the host cell to take up the optimally sized particles that exist when the particles are reduced to an average length of 300 nm or less. In a preferred embodiment, the reduction in particle size is effected by allowing the transfection mixture to slowly lower its pH as a result of the C0 2 and lactate production of the host cells in culture. The gradual decrease in pH causes the transfection mixture to change from oversaturation of Ca 2* and P0 4 3" to an undersaturation of Ca 2* and P0 4 3" , and the resulting slow rate of particle size reduction maximizes the time period during which host cells are exposed to optimally sized particles. In another embodiment, the initial resolvation of overgrown particles in the host cell culture is followed by repeated cycles of admixing fresh overgrown precipitate to the transfection mixture and dissolving the precipitate in the transfection mixture until the desired level of transfection is obtained. Also encompassed herein are methods wherein the co-precipitation occurs in the host cell culture (as described in Section III above) and is allowed to continue until the particles are larger than the optimal size. In these embodiments, the overgrown particles are reduced back down to the optimal size by changing the culture conditions, and after optimal particle size is reached the culture conditions are changed again to permit slow growth of the particles and allow the host cell to take up the particles to form a transfected cell. In addition, all of the methods descnbed herem can be modified to incorporate a continuous cycle of particle overgrowth/particle reduction during mcubation of the transfection mixture designed to maximize the amount of time that the host cells are exposed to optimally-sized particles In a preferred embodiment using a suspension culture in a computer-controlled bioreactor, conditions m the transfection mixture are automatically altered accordmg to an algonthm that estimates particle size and calculates the timing and degree of condition changes needed to achieve maximum cell contact with optimally-sized particles Further details of the mvention can be found m the following examples, which further define the scope of the mvention All citations throughout the specification and all references cited therein are hereby expressly incorporated by reference m their entirety EXAMPLE 1 Matenals and Methods 1 Precipitation Vanous amounts of purified plasmid DNA of the β-galactosidase expression vector pSVβ (Clontech, Palo Alto, C A) or of the tPA expression vector descnbed in Refino et al , Thrombosis and Hematosis, 70 313- 319 (1993) were diluted in a TE buffer (1 mM Tnsφydroxymethyl)amιnomethane (Tπs), 0 1 mM ethylenediaminetetraacetic acid (EDTA), pH 7 6) solution containing concentrations of CaCl 2 that vaned from 250 mM to 500 mM One volume of the DNA/CaCl 2 solution was quickly added to one volume of a phosphate solution (50 mM HEPES, 280 mM NaCl, 1 5 mM Na 2 HP0 4 , pH 7 05) and the mixture was gently vortex mixed to initiate the formation of the precipitate After vanous time periods of mcubation at vanous temperatures, 300 μl ahquots of the mixture were microcentnfuged for 45 seconds and the optical densities of the supematants were measured at 260 nm (OD260) to quantify the amount of unbound DNA (an OD260 of 1 0 corresponded to a DNA concentration of 50 μg/μl ) 2 Transfection CHO cells (CHO-DUKX DHFR rnmus, Urlaub and Chasm, Proc Natl Acad Sci USA, 11 4216-4220 (1980)) and human kidney 293 cells (ATCC CRLl 573) Cells for transfections were passaged regularly every 3 to 4 days by subcultivation m T-flasks or m 500 ml spinner flasks using PS04 medium, a medium propnetary to Genentech, Inc (developed from the onginal formulations of Dulbecco's modified Eagle medium and Ham's F12 medium, catalog numbers 430-3000EB and 430-1700EB m the 1990 catalog of Gibco BRL, Gaithersburg, MD 20877), containmg 2% to 10% fetal calf serum Cell cultures were conducted accordmg to the general methods descπbed m Tissue Culture Laboratory Procedures, Doyle, Griffiths and Newell, eds, J Wiley and Sons, New York (1992) One day pnor to transfection, cells taken from exponentially growing cultures were seeded mto 12 well plates at a density of 2x10 s cells per well, 1 milh ter (ml) of growth medium supplemented with 2% to 10% fetal calf serum was added to each well, and the cell culture plates were mcubated at 37° C under a 5% C0 2 atmosphere for 24 hours The precipitation mixture was added to the individual wells (after vanous time periods of mcubation at vanous temperatures) in volume volume (v/v) ratios that resulted m a final Ca 2 * concentration of 12 5 mM This maintained the condition of Ca 2+ and PO., 3" supersaturation if the pH was held at or above 7 2 CHO cells were exposed to the precipitate for 3 to 6 hours, shocked for 30 seconds with 15% glycerol, washed once and then mcubated m fresh growth medium for 1 to 6 days 293 cells were exposed to the precipitate for 3 to 20 hours and then mcubated m fresh growth medium, without a glycerol shock, for 1 to 6 days Results and Discussion A Physico-chemical parameters of calcium phosphate DNA co-precipitate 1 Temperature Although the heretofore disclosed calcium phosphate transfection protocols provide that DNA-calcium phosphate co-precipitation be earned out for at least 10 minutes, and most protocols recommend 20 minutes of co-precipitation, it was determined that under standard conditions (125 mM Ca 2 *, 0 75 P0 4 ' , 25 μg ml DNA, 20° C, pH 7 05) more than 95% of the DNA bound to the precipitate m less than 1 mmute after admixture of the DNA/CaCl 2 solution to the phosphate solution (see the data for 25 μg/ml DNA m Fig 2) To facilitate an investigation of the effect of temperature, the rate of crystal growth was reduced by using a lower phosphate concentration (0 6 mM P0 4 3' ) The reduced rate of crystal growth permitted the binding of DNA to the precipitate to be observed over a penod of 20 minutes at temperatures between 0° C and 37° C (Fig 1 ) At 37° C, 100% of the DNA bound to the precipitate within 1 mmute whereas at 0° C, all of the DNA remained in solution for 20 minutes This result is due to the higher solubility of calcium phosphate at lower temperatures, which yields a less oversaturated solution with a lowered frequency of crystal nucleation 2 DNA concentration Since DNA concentration affects the transfection efficiency, the influence of DNA concentration on DNA-calcium phosphate co-precipitation was determmed (Fig 2) The precipitation was earned out at 20° C with a phosphate concentration of 0 75 mM The results demonstrated that an increase m the DNA concentration from 25 μg/ml to 50 μg/ml dramatically reduced the binding capacity of the precipitate (Fig 2) At a concentration of 50 μg/ml, over 90% of the DNA remained in solution after 20 minutes and almost no pellet was found upon microcentπfugation for 45 seconds 3 Calcium concentration Although Ca 2 * was present at a concentration that was 170-fold greater than the concentration of P0 4 3 in the precipitation mixture, a further mcrease m Ca 2* strongly affected the formation of a precipitate (Fig 3) In the presence of 250 mM Ca * and 50 μg/ml DNA, 30 to 45 μg/ml DNA was bound to a precipitate Given that the absolute amount of precipitate was limited by the phosphate concentration (0 75 mM), a maximum of 125 μg of precipitate per ml was expected if all phosphate precipitated The precipitate was found to bmd up to 50 μg/ml of DNA in the presence of 375 mM Ca 2 * and 75 μg/ml DNA Thus, DNA can account for approximately 40% of the mass of a precipitate that is formed under optimal conditions B Transient Expression m CHO and 293 Cells The DNA-calcium phosphate co-precipitation was earned out under standard conditions (25 μg/ml DNA, 125 mM Ca 2 *, 075 PO., 3" ) After mcubation at 20° C for vanous time penods, the precipitate was admixed to an exponentially growing cell culture The transfection efficiency was analyzed either m an lntracellular β- galactosidase expression system wherem expressmg cells were identified by X-gal staining according to the method of Somes et al , EMBO, 5 3133-3142 (1986) or m a tPA expression system wherem secreted tPA product was quantified with an ELISA assay according to the method of Bennet et al ,J Biol Chem , 266 5191- 5201 ( 1991 ) As shown in Figs 4 and 5, a one minute precipitation step produced the highest DNA transfection efficiency and expression level among the precipitation mcubation penods tested Inspection of the precipitates under a light microscope (phase contrast, 20X magnification) after 4 hours of exposure to the host cells showed that 1 minute of precipitation resulted m the formation of an enormous number of very small particles whereas 20 minutes of precipitation resulted m the formation of bigger but much fewer particles The high transfection efficiency of the one-minute precipitates in both 293 cells and CHO cells is consistent with the above-descπbed data showing that 1 minute of precipitation was sufficient to bmd 100% of the DNA m a precipitation mixture with 25 μg/ml DNA, 125 mM Ca 2 * and 0 75 mM P0 4 3" High levels of transient expression were achieved by using a Ca 2* concentration of 250 mM (which maximized the frequency of the crystal nucleation event) and a DNA concentration of 50 μg/ml (which saturated the growing crystals) The crystal size was controlled by admixmg the precipitate to the cells after very short incubation penods as shown m Fig 6 Under these conditions, tPA product titers obtained in CHO host cells were as high as 2 μg/ml, and were routinely above 1 μg/ml, 3 days after transfection 293 cells also produced higher transient expression levels after transfection with precipitates formed m 30 seconds with 250 mM Ca 2* and 50 μg/ml DNA Under these conditions, 293 cells yielded 5 μg/ml of tPA 2 days after transfection with a transient specific productivity of about 2 μg 10 cells/day EXAMPLE 2 Matenals and Methods In a first matnx plate test, vanous concentrations of P0 4 (between 04 mM and 2 5 mM) were combined with vanous concentrations of Ca * (between 2 mM and 10 mM) m 1 ml of 140 mM NaCl, 30 mM HEPES, pH 7 2 The plates were mcubated overnight at 37° C Each well was examined under a light microscope (phase contrast, 20X magnification) and scored for the presence of a precipitate In a second matnx plate test, vanous concentrations of Ca * (between 4 2 mM and 18 0 mM) were combined with 1 ml of 30 mM HEPES and 0 95 mM P0 4 3 m PS04 medium at a vanous pH's (between 6 8 and 76) The mixtures were mcubated overnight at 37° C Each well was examined under a hght microscope (phase contrast, 20X magnification) and scored for the presence of a precipitate Results and Discussion The data produced in the first set of tests are summarized m Fig 7 The data points on the curve m Fig 7 represent the lowest concentrations of Ca 2* and PO^ that produced a precipitate Thus, the data points appeanng in Fig 7 define a calcium phosphate solubility curve as a function of Ca 2 * concentration and P0 4 3" concentration Usmg a regression with the method of least squares, the solubility of calcium phosphate as a function of Ca 2 * concentration and P0 4 3 concentration was found to be approximated by the equation y = (4 704)(x * ° K699 ), where y is the Ca 2+ concentration and x is the P0 4 3" concentration At any pomt (x,y) above the curve, the calcium phosphate particles are capable of continued growth At any pomt (x,y) below the curve, the precipitate quickly redissolves The data produced m the second set of tests are summarized m Fig 8 The data points on the curve m Fig 8 represent the lowest Ca 2* concentrations and pHs that produced a precipitate Thus, the data points appearing in Fig 8 define a calcium phosphate solubility curve as a function of Ca * concentration and pH Using a regression with the method of least squares, the solubility of calcium phosphate as a function of Ca 2 * concentration and pH was found to be approximated by the equation y = (8688612)(x "14075 ), where y is the Ca 2* concentration and x is the pH At any pomt (x,y) above the curve, the calcium phosphate particles are capable of continued growth At any pomt (x,y) below the curve, the precipitate quickly redissolves EXAMPLE 3 Mateπals and Methods 1 Precipitation 0 25 mg of purified plasmid DNA of the expression vector pRK7-T103N,N1 17Q,KHRR(296- 299)AAAA tPA (see WO 93/24635 published December 9, 1993), referred to herem as "pRK TNK tPA", was diluted in 2 5 ml of TE buffer containmg 250 mM CaCl- The DNA/CaCl 2 solution was quickly added to 2 5 ml of a phosphate solution containmg 1 5 mM to 4 5 mM P0 4 3 , 50 mM HEPES, pH 7 05 and the mixture was gently vortex mixed to mitiate the formation of the precipitate and mcubated for 1 mmute at room temperature 2 Transfection 293 cells (ATCC CRLl 573) for transfections were passaged as descπbed m Example 1 above Cells were taken from exponentially growmg suspension cultures and seeded m 100 ml of fresh medium at a density of 0 12% packed cell volume (PCV) (approximately 0 4-0 5 x 10 6 cells/ml) m a 100 ml spinner flask Cells were denved from the growmg suspension culture by "medium exchange" wherem an aliquot of suspension culture was centnfuged and pelleted cells were resuspended m fresh medium at the desired cell density The resuspended cells were held m suspension m the spinner flask by a magnetically dnven impeller set at about 70 rpm Approximately 2 hours after seeding cells, the CaCl 2 concentration m the spinner flask was increased (from 1 mM standard concentration of CaCl 2 m medium) to a concentration selected from the range of 6 5 mM to 14 5 mM Following the 1 mmute mcubation of the DNA/calcium phosphate co-precipitate descπbed m step 1 above, 2 ml of the co-precipitate mixture was quickly added to the spinner flask to form the transfection mixture with an initial DNA concentration of 1 mg/hter of medium and an initial Ca 2 * concentration of 9 mM to 17 mM The transfection mixture was mixed well and mcubated for 4 hours at 37° C with agitation by a magnetically dnven impeller set at about 70 rpm The transfection mixture was divided mto two equal parts (A) and (B) Part (A) was mixed with one volume of fresh medium, and part (B) was mcubated for an additional 16 hours under unchanged conditions and then mixed with one volume of fresh medium Following the dilution with fresh medium, the mixtures were mcubated at 37° C with agitation for a total of 27 hours or 52 hours The culture supernatant was harvested by centπfugation Transfection efficiency was analyzed by quantification of secreted TNK tPA product m the culture supernatant using an ELISA standardized to native-sequence tPA (see EPO Pat Publ 93,619) Results and Discussion The data produced are summarized m Figs 9-10 Fig 9 displays the optimization of transient expression with respect to P0 4 3' concentration (with a DNA concentration of 50 μg/ml and a Ca 2* concentration of 125 mM) in the precipitation step and mcubation time m the transfection step As shown m Fig 9, the transient expression level is optimized at a P0 4 3" concentration of 1 125 mM m the precipitation step and an mcubation time of 4 hours m the transfection step Under these conditions, 293 cells yielded 500 ng/ml of TNK tPA 52 hours after transfection with a transient specific productivity of about 500 ng/10 6 cells/day Fig 10 displays the optimization of transient expression with respect to the Ca 2* concentration and incubation time in the transfection step (with 125 mM Ca 2 *, 1 125 mM P0 4 3 and 50 μg/ml DNA in the precipitation step) As shown Fig 10, the transient expression level is optimized with an mcubation time of 4 hours m the transfection step Under these conditions, 2$3 cells yielded 500-750 ng/ml of TNK tPA 52 hours after transfection with a transient specific productivity of about 500-800 ng 10 6 cells/day 9 mM, 12 5 mM and 17mM Ca concentrations m the transfection step produced roughly equivalent transient expression levels The maximum titers obtained usmg 100 ml spinner flasks (under the conditions descnbed above for a 4 hour transfection mcubation) or usmg 250 ml spinner flasks (with a modification of the above-descnbed conditions wherem 100 ml of transfection mixture was mcubated for 4 hours and then mixed with 1 volume of fresh medium) were 3 mg/L of TNK tPA after 5 days and 1 5 mg/L after 2 days The maximum transient specific productivity obtained was about 1 mg of TNK tPA/10 6 cells/day EXAMPLE 4 Materials and Methods 1 Precipitation 1 2 mg of purified plasmid DNA of the expression vector pRK DNase 3 (descnbed m WO 90/07572 published July 12, 1990) was diluted m 30 ml of TE buffer containing 500 mM CaCl 2 The DNA/CaCl 2 solution was quickly added to 30 ml of a phosphate solution containmg 1 5 mM P0 4 3 , 50 mM HEPES, pH 7 05 in a 200 ml flask with gentle shakmg to mitiate the formation of the precipitate and mcubated for 1 -2 mmutes at room temperature 2 Transfection 293 cells (ATCC CRLl 573) for transfections were passaged as descπbed m Example 1 above Two days pnor to transfection, cells were taken from exponentially growmg suspension cultures and seeded 1 liter of fresh medium at a density of 005% packed cell volume (PCV) m a 2 liter bioreactor Cells were deπved from the growmg suspension culture by "medium exchange" wherem an aliquot of suspension culture was centnfuged and pelleted cells were resuspended m fresh medium at the desired cell density The resuspended cells were held in suspension m the reactor by a mechanically dnven impeller set at about 70 m and mcubated at 37° C until a cell density of about 0 2% PCV (approximately 06-08 x 10 6 cells/ml) is reached Following the 1 -2 mmute mcubation of the DNA/calcium phosphate co-precipitate descnbed m step 1 above, the co-precipitate mixture was drawn mto a 60 ml syπnge and injected directly mto the reactor to form the transfection mixture with an initial DNA concentration of 1 2 mg/liter and an initial Ca 2+ concentration of 15 mM The transfection mixture was mcubated m the reactor for 3 5 hours at 37° C with agitation as descnbed above. One volume of fresh medium was added and the culture was incubated at 37° C with agitation for a total of 74 hours. Culture samples were taken at 19, 24, 43, 50 and 74 hours post transfection. The culture supernatant was harvested by centrifugation. The secreted DNase product in the culture supernatant was quantified by a sandwich ELISA method. Microtiter plates were coated with goat anti-recombinant human DNase antisera, coated plates were contacted with culture supernatant, plates were washed to remove unbound DNase, and bound DNase was detected with rabbit anti-recombinant human DNase antisera conjugated to horse radish peroxidase (HRP). Results and Discussion Table I below contains time course data for transient expression of DNase versus PCV post-transfection. Table I Transfection of 293 HEK with pRK DNase 3 hours post transfection PCV DNase(μg/ml) 0 0.19 0 19 0.27 0.25 24 0.35 0.53 43 0.42 1.2 50 0.5 1.4 74 0.55 2.2 The data of Table I are displayed graphically in Fig. 11. As shown Table I and in Fig. 11 , the transfected cell culture produced 0.25 μg/ml DNase with 0.27% PCV at 19 hours and 2.2 μg/ml DNase with 0.55% PCV at 74 hours post transfection, yielding a specific productivity of about 500 ng/10 6 /day. EXAMPLE 5 Materials and Methods 1. Precipitation 20 mg of purified plasmid DNA of the expression vector pRK TNK tPA (WO 93/24635) is diluted in 200 ml of TE buffer ∞nuiining 250 mM CaCl 2 . The DNA/CaCl 2 solution is mixed inline with 200 ml of a phosphate solution containing 2.25 mM P0 4 3' , 50 mM HEPES, pH 7.05 by means of a pump with two pump heads and two tube lines that merge prior to emptying into a 40 L bioreactor. The pump rate and the length of the tubing from the point of inline mixing to the bioreactor entry port are adjusted to achieve a 30 second incubation of the precipitation mixture prior to dilution in the reactor cell culture. 2 Transfection 293 cells (ATCC CRLl 573) for transfections are passaged as descnbed m Example 1 above Two days pnor to transfection, cells are taken from exponentially growmg suspension cultures and seeded in 20 liters of fresh medium at a density of 0 05% packed cell volume (PCV) m a 40 liter bioreactor Cells are deπved from the growmg suspension culture by "medium exchange" wherem an aliquot of suspension culture is centnfuged and pelleted cells are resuspended m fresh medium at the desired cell density The resuspended cells are held in suspension m the reactor by a mechanically dnven impeller set at about 50 m and mcubated at 37° C until a cell density of about 0 2% PCV (approximately 0 6-0 8 x 10 cells/ml) is reached At 0 2% PCV, the pH is increased to 7 2-7 5 and 800 ml of 250 mM CaCl 2 is added to mcrease the Ca 2 * concentration by 7 5 mM Following the 30 second mcubation of the DNA/calcium phosphate co-precipitate descnbed in step 1 above, the co-precipitate mixture is emptied mto the reactor to form the transfection mixture with an initial DNA concentration of 1 0 mg/ ter The transfection mixture is mcubated m the reactor for 5 hours at 37° C with agitation as descπbed above After 5 hours of mcubation, 20 L of fresh medium is added and the culture is incubated at 37° C with agitation for a total of 3 days The culture supernatant is harvested by centnfugation and/or filtration Transfection efficiency is analyzed by quantification of secreted TNK tPA product in the culture supernatant usmg the standard tPA ELISA method descnbed m EPO Pat Publ 93,619

Claims: WE CLAIM 1 A method for introducing a desired nucleic acid mto a eukaryotic host cell, compnsmg (a) admixmg Ca 2+ , PO< 3 and the desired nucleic acid to form a precipitation mixture, (b) mcubatmg the precipitation mixture to form particles compnsmg calcium phosphate and the desired nucleic acid, and allowing the particles to grow to an average length of up to about 300 nm, (c) performing a step selected from the group consisting of (1) diluting the precipitation mixture and simultaneously admixmg the precipitation mixture with a eukaryotic host cell lackmg a cell wall to form a transfection mixture wherem the particles are capable of growth at a rate that is substantially lower than the rate at which the particles grew m the precipitation mixture m step (b), and (2) diluting the precipitation mixture to form a diluted precipitation mixture, and thereafter admixmg the diluted precipitation mixture with a eukaryotic host cell lackmg a cell wall to form a transfection mixture wherem the particles are capable of growth at a rate that is substantially lower than the rate at which the particles grew in the precipitation mixture m step (b), and (d) mcubatmg the transfection mixture to allow the eukaryotic host cell to take up the particles to form a transfected cell 2 The method of claim 1 wherem m step (b) the particles are allowed to grow to an average length of up to about lOO nm 3 The method of claim 1 wherem the eukaryotic host cell is a mammalian cell The method of claim 1 wherem m step (b) the precipitation mixture is mcubated for a penod of up to about 60 seconds 5 The method of claim 4 wherem in step (b) the precipitation mixture is mcubated for a penod of up to about 30 seconds The method of claim 1 wherem the desired nucleic acid is DNA The method of claim 6 wherem m step (a) the precipitation mixture compnses an initial Ca 2* concentration of about 250 mM The method of claim 7 wherem m step (a) the precipitation mixture compnses an initial concentration of the desired DNA of about 50 μg/ml The method of claim 1 wherem in step (c) the transfection mixture contains an initial Ca 2+ concentration that is at least about ten fold lower than the initial Ca 2* concentration of the precipitation mixture m step (b) 10 A method for mtroducmg a desired nucleic acid mto a eukaryotic host cell m suspension, compnsmg (a) admixmg Ca 2* , P0 4 3 and the desired nucleic acid to form a precipitation mixture, (b) mcubatmg the precipitation mixture to form particles compnsmg calcium phosphate and the desired nucleic acid, and allowing the particles to grow to an average length of up to about 300 nm, (c) performing a step selected from the group consisting of (1) diluting the precipitation mixture and simultaneously admixmg the precipitation mixture with a mixture containing in suspension a eukaryotic host cell lackmg a cell wall to form a transfection mixture wherein the particles are capable of growth at a rate that is substantially lower than the rate at which the particles grew m the precipitation mixture m step (b), and (2) diluting the precipitation mixture to form a diluted precipitation mixture, and thereafter admixmg the diluted precipitation mixture with a mixture containmg m suspension a eukaryotic host cell lackmg a cell wall to form a transfection mixture wherem the particles are capable of growth at a rate that is substantially lower than the rate at which the particles grew m the precipitation mixture m step (b), and (d) mcubatmg the transfection mixture to allow the eukaryotic host cell to take up the particles to form a transfected cell in suspension 11 The method of claim 10 wherem m step (b) the particles are allowed to grow to an average length of up to about lOO nm 12 The method of claim 10 wherem m step (b) the precipitation mixture is mcubated for a penod of up to about 2 mmutes 13 The method of claim 12 wherem m step (b) the precipitation mixture is mcubated for a penod of up to about 1 mmute 14 The method of claim 13 wherem m step (b) the precipitation mixture is mcubated for a penod of up to about 30 seconds 15 The method of claim 10 wherem m step (c) the transfection mixture compnses a total volume of at least about 1 liter 16 The method of claim 15 wherem m step (c) the transfection mixture compnses a total volume of at least about 20 liters 17 The method of claim 10 wherem the eukaryotic host cell is a mammalian cell 18 The method of claim 10 wherem the desired nucleic acid is DNA 19 The method of claim 18 wherem m step (a) the precipitation mixture compnses an mitial PO, 3 concentration of about 1 125 mM 20 The method of claim 19 wherem m step (a) the precipitation mixture compnses an mitial concentration of the desired DNA of about 50 μg/ml 21 The method of claim 10 which after step (d) further compnses the step of (e) dissolving the particles m the transfection mixture and growmg the transfected cell m suspension 22 The method of claim 10 wherem m step (c) the Ca 2* concentration m the mixture compnsmg a eukaryotic host cell m suspension is mcreased to about 7 5 mM before the precipitation mixture or diluted precipitation mixture is admixed with the mixture compnsmg a eukaryotic host cell in suspension 23 The method of claim 10 wherem in step (a) the desired nucleic acid, Ca + , and P0 4 3 flow into an mtake pipe, m step (b) the precipitation mixture is mcubated m the mtake pipe, and m step (c) the mtake pipe empties the precipitation mixture mto a culture vessel containmg the mixture compnsmg a eukaryotic host cell m suspension 24 The method of claim 10 wherem m step (a) the desired nucleic acid, Ca 2* and P0 4 3 flow mto an mtake pipe, m step (b) the precipitation mixture is mcubated m the mtake pipe, m step (c) diluent flows mto the precipitation mixture through another mtake pipe at a pomt downstream of the nucleic acid, Ca 2* , and P0 4 3 mtake and dilutes the precipitation mixture to form the diluted precipitation mixture, and thereafter m step (c) the mtake pipe empties the diluted precipitation mixture mto a culture vessel containmg the mixture compnsmg a eukaryotic host cell m suspension 25 The method of claim 10 wherem m step (c) the transfection mixture contains an mitial Ca 2* concentration that is at least about ten fold lower than the mitial Ca 2+ concentration of the precipitation mixture step (b)

Inventors: JORDAN MARTIN, WURM FLORIAN M, JORDAN, MARTIN, WURM, FLORIAN, M.
Applicants: GENENTECH INC, GENENTECH, INC.
Application year: 1995
Main classification: C12N15
IPC codes: C12N5, C12N15