A METHOD FOR CULTURING OF CELLS ON A MEMBRANE

Abstract: There is disclosed a method of culturing cells which produce products and identifying the thereby produced products, characterised in that the cells are cultured on the surface of a quantity of growth medium, which surface has been divided into a plurality of growth areas thereby limiting the amount of growth-medium which is available to the cells so that these reach a metabolic stage in which said products are produced, and identifying the thereby produced products by an in situ screening procedure.

Description: A method for culturing of cells on a membrane. This invention relates to cell culture. It is to be understood that the terms "growth" and "proliferation" when applied to cells are synonymous for the purposes of this disclosure. Both pro aryotic and eukaryotic cells are capable of producing materials of immense pharmaceutical and industrial value. These materials may be monoclonal antibodies produced by eukaryotic hybridoma cells, or products of genes of eukaryotic origin expressed in prokaryotic cells and secreted or otherwise obtained therefrom- or indeed products of prokaryotic origin, properly expressed by and secreted from such organisms without the intervention of manipulative recombinant nucleic acid methodology. The metabolic state of such cells is important in determining the production and excretion of such products. For example, many common micro-organisms generally only produce and secrete such products when they have been induced to enter a phase of growth-arrest, which may be elicited by, amongst other things, nutrient depletion, competition with other micro-organisms either for nutrients or oxygen, environmental changes in the microbial habitat or by the production of anti-metabolites from a different strain of micro-organism growing in the vicinity. Examples of products which growth-arrested micro-organisms produce are antibiotics, anti-tumour agents such as mitomycin C, citrates, and heat stable enzymes which can, for example, be used in washing powders. Initial screening procedures for cultures of micro-organisms which are potentially capable of producing such products may involve inoculating a suitable growth-medium with a crude suspension of, for example, soil. The growth medium is incubated under appropriate conditions and mixed cultures of micro-organisms are obtained. Unless these are induced to enter growth-arrest they generally will not produce, or at least excrete, appreciable quantities of useful microbial products. Moreover, in such screening procedures, a considerable quantity of growth-medium and a large number of culture vessels can be wasted through colonies not being growth-arrested and thus over-growing themselves, or over-growing nearby colonies. Conventionally, colonies of micro-organisms, including mixed colonies containing a number of strains which additionally may have different growth rates on the same medium, reach growth-arrest slowly, but once arrested yield a dense heterogeneous mass from which it is difficult, if not impossible, to identify strains of micro-organisms of interest by use of an .in situ screening procedure. According to the present invention there is provided a method of culturing cells which produce products encouraging the cells to produce the products and identifying the thereby produced products, characterised in that the cells are cultured on the surface of a quantity of growth medium, which surface has been divided into a plurality of growth areas thereby limiting the amount of growth-medium which is available to the cells so that these reach a metabolic stage in which said products are produced, and identifying the thereby produced products by an .in situ screening procedure. The growth medium available to the cells may be limited by bounding the area of surface of growth medium available to the cells. The area of surface of growth medium may be bounded by a membrane in contact with the surface. The membrane may comprise a plastic film material which may be substantially impermeable to oxygen. The plastics film material may be polyethene. The membrane may, however, comprise a metallic material. The membrane may have a plurality of spaced apart apertures defining areas of growth medium surface on which the cells can be grown. The membrane may comprise a backing-layer which serves to rigidify same. The membrane may be multilaminate in which a plurality of membranes may be superimposed upon and bonded to a semipermeable sheet. Said sheet may itself be multilaminate and may be impermeable to proteins and molecules of like-size. The membrane may be adapted to be retained in a Petri dish or any other suitable container used in cell culture, whether such culture be of bacterial, animal or plant cells. Preferably the apertures define an area of between 0.8 and 113 square millimetres. More preferably the apertures define an area of between 0.8 and 51 square millimetres and most preferably the apertures define an area of between 3 and 13 square millimetres. The apertures may be substantially of circular cross section and may be spaced between 6 and 10 milimetres apart. The apertures may be positioned on the membrane in rows or columns, whereby a regular square arrangement of apertures in the membrane may be provided. Each membrane may be uniquely identifiable, for example, by being numbered serially. The serial number may be applied to the membrane in a non-toxic coloured material printed onto the surface of the membrane which contacts the medium and is thereby partially transferred thereto by which the growth-medium likewise may be numbered. The membrane may comprise means for orientating same by which, once the membrane has been removed from , ,Λ - β , 10056 - 6 - the surface of the growth medium, colonies of cells remaining attached to the membrane at the periphery of the apertures therein can be located from the corresponding colonies remaining on the growth medium. Said means may be provided by cut out sections in the membrane. The membrane may have a projection to facilitate aseptic lifting and lowering thereof on to the growth medium surface. Preferably, however, said means comprises a grid of a non toxic coloured material applied to the surface of the membrane which contacts the medium and is thereby partially transferred thereto. A detergent may be added to the coloured material to assist transfer thereof to the growth medium. Said detergent may be a polyoxyethylenesorbitan, preferably the monolaurate derivative thereof. Said non-toxic coloured material may be an Indian ink, such as Pelican Fount Indian Ink. A stack of such membranes may be used to produce multiple copies of cultures of cells. Cells may be grown in the apertures of the membrane which define areas of growth medium. When an area of growth medium is confluent with cell growth the stack of membranes may be dismantled and the membranes may be used to inoculate further supplies of growth medium. A stack of membranes may be treated with a release agent, such as, for example, a siliconising agent. Screening for cell products may comprise applying an indicator agent over colonies thereof and immobilising the indicator agent whereby changes in the indicator agent around said colonies may be detected and indicate the production of said products. The indicator agent may be a micro-organism, or it may be a chemical indicator. Alternatively, and/or additionally, said products may be detected by immunological means whereby, for example, said products react with antibodies or indeed antigens in the case where said products are themselves antibodies. Said antibodies or antigens may be labelled. Said labelling may be with a fluorescent marker, or any other suitable marker. The metabolic stage of the cells may be growth-arrest, and the cells may be prokaryotic. Where the cells are prokaryotic and the metabolic stage is growth arrest, the indicator agent may be a micro-organism such as E. coli which may be immobilised in soft agar, or it may be a chemical agent. The chemical agent may be applied in soft agar or in another manner such as a spray if the chemical reaction between indicator and anti-metabolite, or other microbial product can be detected in the growth medium below the micro-organism. The indicator agent may be a chromogenic substrate added to the medium surrounding the growth arrested micro-organisms. Screening growth arrested micro-organisms for their production of specific enzymes, for example, may comprise applying a mutant indicator cell line, which may be a mutant form of E. coli, in soft agar, over the growth-arrested micro-organisms. The mutation in the cell line may be such that the mutant cells are unable to grow in medium not supplemented with factors, such as enzymes, sugars or amino acids. If the growth-arrested micro-organisms produce factors required for growth of the mutant cell line, the mutant cells will grow and can be detected. The invention will be further apparent from the following description, with reference to the several figures of the accompanying drawings, which show, by way of example only, one form of method embodying the invention, and apparatus for carrying out the method. Of the drawings:- Figure 1 is a section through a Petri dish containing a membrane bounding areas of growth medium; Figure 2 is a plan of the Petri dish illustrated in Figure 1; Figure 3 is a section through another Petri dish containing a stack of membranes; Figure 4 shows the preferred disposition of apertures in a membrane; Figure 5 shows a membrane according to the invention comprising a backing material which contacts the surface of growth medium; and Figure 6 shows a further form of membrane adapted for use in culture of eukaryotic cells, particularly those producing monoclonal antibodies. The drawings illustrate a method of culturing cells so that they can produce identifiable products, characterised in that the cells 10 are cultured on the surface of a quantity of growth medium 11, which surface has been divided into a plurality of growth areas 12 thereby limiting the amount of growth medium 11 which is available to the cells 10 so that the cells 10 reach a metabolic stage in which said products are produced, and identifying the thereby produced products by an .in. situ screening procedure. The growth medium available to the cells is limited by bounding the area of surface of growth medium available to the cells 10. The surface area of growth medium 11 is bounded by a membrane 13 in contact with the surface. The membrane 13 comprises a plastic film material, such as polyethene, which is substantially impermeable to oxygen. Preferably the surface of the membrane, other than the surface which contacts the growth medium, has a waxy characteristic, whereby moisture is substantially prevented from spreading thereon. - li ¬ lt will be appreciated however, that membranes according to the invention comprised by a metallic material, such as an aluminium wafer, might suggest themselves to specific applications. The membrane 13 has a plurality of spaced apart apertures 14 defining the areas 11 of growth medium surface on which the cells 10 are grown. The membrane 13 is adapted to be retained in a Petri dish 15 perhaps of 10 cm diameter or any other suitable container used in cell culture, whether this be bacterial, animal or plant cell culture. The apertures 14 each define an area of between 0.8 and 113 square millimetres, more preferably an area of between 0.8 and 51 square millimetres and most preferably an area of between 3 and 13 square millimetres. As shown in Figure 2, the apertures are substantially of circular cross section. The apertures 14 are spaced between 6 and 10 milimetres apart, although several spacings are contemplated according to the application (see below). The apertures 14 are positioned on the membrane 13 in rows or columns, whereby a regular square arrangement 15 of apertures 14 on the membrane 13 provided, as shown in Figure 4. It is preferred that a face centred square arrangement (quincunx) be avoided in applications where the membranes are treated with a transferable ink for orientating same on the surface of growth medium (see below) . Each membrane is uniquely identifiable, by being numbered serially 16. Such identification is provided in a form which is transferable from the membrane to the surface of the growth medium, and also in a form which is retained on the membrane 13. The serial number 16 is applied to the membrane by a non-toxic coloured material being applied to the surface of the membrane 12 which contacts the medium and which is thereby partially transfered thereto, and by which the growth medium is likewise numbered. The same number is applied to the non-contacting surface of the membrane in an indelible ink. The membrane may be orientated with respect to a surface of the growth medium with which previously it had been in contact whereby colonies of cells remaining thereon can be identified from the corresponding apertures in said membrane. The membrane may be orientated by providing cut out sections 17 in the membrane, which section correspond to particular points on the growth medium surface. The membrane has a projection 28 to facilitate aseptic lifting and lowering of same onto the growth medium surface. Preferably, however, a grid 18 of a non toxic coloured material such as an indian ink (Pelican Fount Indian Ink) is applied to the surface of the membrane 13 which contacts the medium 11 and is thereby partially transfered thereto. Rows and columns of the grid 18 may be lettered or numbered in the same ink. A detergent is added to the ink to aid partial transfer thereof to the the growth medium surface. The detergent is biologically relatively benign, and preferably is the monolaurate derivative of polyoxyethylenesorbitan. The non-contacting surface of the membrane 13 may carry a similar grid, similarly lettered or numbered, in indelible ink. A stack 20 of membranes 13 is used to produce multiple copies of cultures of cells. In use the cells are grown on a growth medium 11 surface until they reach the boundary of surface of growth medium imposed by the apertures in the stack of membranes. Cell growth is arrested by limitation of the medium available to the cells. As the cells fill the apertures the stack of membranes becomes contaminated around the apertures. The stack of membranes is dismantled and the individual membranes used to inoculate further supplies of growth-medium. The stack 20 of membranes 13 is treated with a release agent, such as, for example, a siliconising agent. Screening for cell products comprises application of an indicator agent 19 over colonies thereof and immobilising the indicator agent 19 whereby changes in the indicator agent shown at 19a around said colonies may be detected and indicate the production of said products. The indicator agent 19 may be a micro-organism, or it may be a chemical indicator. Alternatively, or additionally, said products may be detected by immunological means whereby, for example, said products react with antibodies or indeed antibodies or antigens in the case where said products are themselves antibodies. Said antibodies or antigens may be labelled with a fluorescent, or any other suitable marker. In the case where the cells are growth-arrested microbial cells the screening method comprises applying an indicator agent 19 over colonies of growth-arrested micro-organisms and immobilising the indicator agent whereby changes in the indicator agent shown diagrammatically at 19a in Figure 1 around the colonies of growth arrested micro-organisms may be detected to indicate the production of useful microbial products. In practice the indicator agent might be a micro-organism such as E.coli which can be immobilised in soft agar. Anti-metabolites, or other microbial products, could be demonstrated by areas of reduced growth of lysis of the E.coli indicator around the growth arrested product producing micro-organisms 10a. For example, this may appear as a clear area 19a, in the agar containing the E.coli, around the colonies of growth-arrested micro-organisms 10a which are producing products. Where the indicator micro-organisms are able to grow around colonies not producing anti-metabolites the agar will be opaque. It will be appreciated, however, that the indicator could be a chemical agent such as a dye which binds preferentially to microbial products and which can be detected by means of a colourimetric reaction. The chemical agent can be applied in soft agar or in another manner, such as a spray, if the chemical reaction between the indicator and anti-metabolite, or other microbial product can be detected in the growth medium below the micro-organism. Natural products, such as enzymes, produced by the growth arrested micro-organisms, can be detected by addition of a chromogenic substrate to the medium surrounding the growth-arrested micro-organisms. For example, nitrocefin can be added to the growth medium, facilitating the detection of, for example, the microbial production of penicillinase since, when penicillinase reacts with nitrocefin, the nitrocefin, otherwise yellow, turns red. Screening for enzymes and other natural products may comprise applying a mutant indicator cell line, which can be a mutant form of E.coli, in soft agar, over the growth arrested micro-organisms. The mutation in the indicator cell line is such that the mutant cells are unable to grow in medium not supplemented with factors. If the growth-arrested micro-organisms produce factors required for growth of the mutant cell line, the mutant cells will grow and can be visualised. The factors may comprise enzymes, which restore an enzyme deficiency in the mutant indicator cells or they may comprise a metabolite such as a sugar or amino acid which the mutant indicator cells require for growth. In a further embodiment, the indicator cell line may be such that a defined reagent is toxic to, or otherwise inhibits, growth of the indicator cells. The reagent may be added to the soft agar in which the indicator cells are layered onto the growth-arrested micro-organisms. The reagent, for example, may be an antibiotic or chemically modified antibiotic. Microbial products produced by growth-arrested micro-organisms, may cleave, or otherwise modify the reagent, thus enabling the indicator cells to grow. Growth, rather than lysis, of the indicator cells is then evidence of the production by the growth-arrested micro-organisms of a defined class of microbial products. A mutation may be made in the indicator cell line so that the indicator cells become susceptible to the reagents. Mutliple screening of a single colony of growth- arrested micro-organisms can be carried out. By having, for example, an indicator agent in the growth medium and a different indicator agent in soft agar which is then layered on to the growth arrested micro-organisms, it is possible to detect the production of two different microbial products. Use of membranes according to the invention increases the speed of work involving numerous micro-organisms or cells, and increases the likelihood that each aperture contains a pure, or substantially pure culture. Use of the membranes enables automation and standardisation of techniques involving cell culture in industrial processes. Use of the membranes aids automatic inoculation aspects and automatic detection of required cell products by restricting cell growth chiefly to designated positions, which are explicitly numbered (or have implied numbered positions by row and column) . Where the cells seeded into the apertures in the membranes are eukaryotic, for example antibody-producing hybridomas, the membrane preferably is positioned on the surface of a collagen or gelatin substrate, although any other suitable solid support media may be used, including fibrin and agar or agarose gels, or suitable derivatives thereof. In practice, the membrane is placed onto the solid support medium which contains suitable nutrients. A mixed population of antibody-producing cells is then inoculated into the medium, at a dilution which provides for an average of rather less than one viable cell per aperture. The cells are grown and the membrane is then removed to a second growth medium so as to preserve alive the various clones adhering to the periphery of the apertures in the membrane, and the first medium is tested for a cell product. Where the product is an antibody, the medium at the position of previous aperture sites is detected by applying a labelled antigen thereto. The antigen is labelled with a fluorescent dye, but more conveniently an indirect method is used in which antigen is added at the sites of the apertures, and second antibodies raised in an immunologically distinct organism against the antigen are fluorescently labelled and bound thereto. The desired clones are located from the corresponding aperture on the membrane now in the second growth medium, purified if necessary and grown in bulk. The membrane 71 may comprise a backing layer 72 which serves to rigidify same as shown in Figure 5. Should such cells not be capable of suitable growth on said solid growth media, a modified mulilaminar membrane 21 shown in Figure 6 is provided. This comprises a first layer comprised by a polyethene membrane 31 of relatively increased thickness so that the apertures 14 define shallow wells 32. This is superimposed upon and bonded to a semiper eable sheet 41 having apertures 14 of size and configuration corresponding to those in the first layer 31, and a pore size of perhaps 0.5 micrometres. The layer 41 is itself bonded to a further layer 51 of similar material but lacking apertures. The layer 51 is further bonded to a sheet of material 61, such as cellophane, which allows passage thereacross of small molecules such as water and vitamins etc, but prevents passage thereacross of molecules such as proteins, and particularly antibodies. The membrane 21 comprising four layers is floated on a liquid nutrient medium, and is inoculated with cells as above, viz, at a density which gives rather less than one viable cell per aperture 14. Clones of cells result in the apertures 14, and antibodies produced thereby diffuse into the third laye 51 of the membrane 21 having a pore size of 0.5 microns but no apertures. The antibodies do not diffuse into the fourth layer 61, viz, that comprised by the cellophane-like material. After appropriate cell growt the upper membranes are transferred to a second growth medium and the lower two membranes are tested for the presence of antibodies. The cell clones producing the desired antibody are then isolated from the corresponding apertures in the upper layers of the membrane, now in the second growth medium. It will be appreciated that it is not intended to limit the invention to the above examples only, many variations, such as might readily occur to one skilled in the art, being possible, without departing from the scope thereof as defined by the appended claims. For example, the multilaminate membrane can be adapted for the detection in situ of nucleic acid molecules. In this case, the upper membrane has pores large enough to enable some cells, including prokaryotic cells where appropriate, to grow downwardly into the lower layer adjacent the cellophane or like-material layer. The upper two layers comprised by the membrane are then removed and transferred to a second growth medium, whilst the cells in the lower layer are denatured and nucleic acid therein exposed to known probing regimes. The upper layers in the multilaminate membrane may be coated with fibronectin, and like molecules to encourage cell growth. Cells identified as useful by use of the method can be grown according to known means and the products identified by the method purified therefrom.

Claims: CLAIMS 1. A method of culturing cells which produce products and encouraging the cells to produce products and identifying the thereby produced products, characterised in that the cells are cultured on the surface of a quantity of growth medium, which surface has been divided into a plurality of growth areas thereby limiting the amount of growth-medium which is available to the cells so that these reach a metabolic stage in which said products are produced, and identifying the thereby produced products by an in situ screening procedure. 2. A method according to claim 1, characterised in that the growth medium available to the cells is limited by bounding the area of surface of growth medium. 3. A method according to claim 2, characterised in that the area of surface of growth medium is bounded by a membrane in contact with the surface. 4. A method according to claim 3, characterised in that the membrane comprises a plastics film material which is substantially impermeable to oxygen. 5. A method according to claim 3 , characterised in that the plastics film material is polyethene. 6. A method according to claim 3, characterised in that the membrane comprises a metallic material. 7. A method according to any one of claims 3 to 6, characterised in that the membrane comprises a plurality of spaced apart apertures defining areas of growth medium surface on which the cells can be grown. 8. A method according to claim 3, characterised in that the membrane is multilaminate in which a plurality of membranes are superimposed upon, and bonded to, a semipermeable sheet. 9. A method according to the preceding claim, characterised in that said sheet is multilaminate and is impermeable to proteins and molecules of like-size. 10. A method according to any one of claims 3 to 8, characterised in that the membrane is adapted to be retained in a Petri dish. 11 A method according to any one of claims 7 to 10, characterised in that the apertures are substantially of circular cross section and define an area of between 0.8 and 113 square millimetres. 12. A method according to any one of claims 7 to 10, characterised iri that the apertures are substantially o circular cross section and define an area of between 0.8 and 51 square millimetres. 13. A method according to any one of claims 7 to 10, characterised in that the apertures are substantially of circular cross section and define an area of between 3 and 13 square millimetres. 14. A method according to any one of claims 7 to 13, characterised in that the apertures are spaced between 6 and 10 milimetres apart. 15. A method according to any one of claims 7 to 14, characterised in that the apertures are positioned on the membrane in rows or columns, whereby a regular square arrangement of apertures in the membrane is provided. 16. A method according to any one of claims 3 to 15, characterised in that the membrane is uniquely identifiable, for example, by being numbered serially. 17. A method according to claim 16, characterised in that the serial number is applied to the membrane in a non-toxic coloured material onto the surface of the membrane which contacts the medium and is thereby partially transferred thereto by which the growth-medium likewise is numbered. 18. A method according to any one of claims 3 to 17, characterised in that the membrane comprises means for orientating same by which, once the membrane has been removed from the surface of the growth medium, colonies of cells remaining attached to the membrane at the periphery of the apertures therein can be located from the corresponding colonies remaining on the growth medium. 19. A method according to the preceding claim, characterised in that said means are provided by cut-out sections in the membrane. 20. A method according to claim 18, characterised in that said means comprises a grid of a non toxic coloured material applied to the surface of the membrane which contacts the medium and is thereby partially transferred thereto. 21. A method according to claim 20, characterised in that rows and columns of said grid are numbered or lettered 22. A method according to the preceding claim, characterised in that a detergent is added to the coloured material to assist transfer thereof to the growth medium. SUBSTITUTE SHEET 23. A method according to the preceding claim, characterised in that said detergent is a polyoxyethylenesorbitan. 24. A method according to any one of claims 20 to 23, characterised in that said non-toxic coloured material is an Indian ink. 25. A method aqccording to the preceding claim, characterised in that said ink is Pelican Fount Indian Ink. 26. A method according to claim 3, characterised in that a stack of such membranes is used to produce multiple copies of cultures of cells, so that when an area of growth medium defined by apertures in the membranes is confluent with cell growth the stack of membranes can be dismantled and the membranes used to inoculate further supplies of growth medium. 27. A method according to claim 26, characterised in that said membranes are treated with a release agent, such as a siliconising agent. 28. A method according to any one of claims 3 to 27, characterised in that the screening for cell products comprises applying an indicator agent over colonies 6 - 28 - thereof located in the apertures of the membranes and immobilising the indicator agent whereby changes in the indicator agent around said colonies may be detected and indicate the production of said products. 29. A method according to the preceding claim, characterised in that the indicator agent is a micro-organism. 30. A method according to claim 28, characterised in that said indicator agent is a chemical indicator. 31. A method according to claim 28, characterised in that said indicator agent is applied in soft agar to the cells. 32. A method according to claim 28, characterised in that said indicator is applied to the cells as a spray where the chemical reaction between indicator and product can be detected in the growth medium below the cells. 33. A method according to any one of claims 30 to 32, characterised in that the indicator agent is a chromogenic substrate added to the medium surrounding the cells. 34. A method according to any claim 28, characterised in that said products are detected by i munological means whereby the products react with antibodies or antigens. 35. A method according to the preceding claim, characterised in that said antibodies or antigens are labelled. 36. A method according to the preceding claim, characterised in that the antibodies or antigens are labelled with a fluorescent dye. 37. A method according to claim 1, characterised in that the metabolic stage of the cells is growth-arrest, and the cells are prokaryotic. 38. A membrane for use in culturing cells and encouraging these to produce products, characterised in that the membrane comprises a plurality of apertures of substantially circular cross section each defining an area of between 0.8 and 113 square millimetres. 39. A membrane according to claim 38, characterised in that it is adapted to be retained in a Petri dish. 40. A membrane according to either of claims 38 or 39, characterised in that the apertures are spaced between 6 and 10 millimetres apart. 41. A membrane according to any one of claims 38 to 40, characterised in that said apertures are positioned on the membrane in rows or columns, whereby a regular square arrangement of apertures in the membrane is provided. 42. A membrane characterised in that it is substantially as described herein. SUBSTITUTE SHEET

Inventors: SNEATH PETER HENRY ANDREW, SNEATH, PETER, HENRY, ANDREW
Applicants: UNIV LEICESTER, UNIVERSITY OF LEICESTER
Application year: 1990
Main classification: C12M1
IPC codes: C12M1, C12M3, C12N1, C12N5, C12Q1