APPARATUS FOR FUSING CELLS

Abstract: Apparatus for fusing cells, wherein the fusion chamber is provided with a pair of spaced electrodes whose opposite sur­ faces have such a vertical dimension as to prevent any nonuniform electric field from adversely affecting fusion of cells. The electrodes may comprise a pair of parallel plate electrodes or concentrically arranged hollow cylindrical electrodes the outer one of which has an inner circumferential surface of a first radius while the inner electrode has an outer circumferential surface of a second radius smaller than the first radius, the difference be­ tween the first and second radii being sufficiently small as compared with the whole length of the radii.

Description: The invention relates to a fusion chamber for fusing cells. Generally, an apparatus comprising such a fusion chamber which encloses a pair of spaced electrodes between which a suspension containing cells of two different species to be fused together are put, comprises also a source of electrical stimulus, means for applying an electrical stimulus to the cells through the electrodes, and a controller for controlling the application of the electrical stimulus to the cells. As the electrical stimulus an alternating current (AC) field and a direct current (DC) pulse are used, and as the electrodes a pair of wire or plate electrodes are used. For effecting cell fusion an AC field is first applied through the electrodes to the cells suspended in a solution put between the electrodes to cause close membrane contact between adjacent cells, and then a DC pulse of a relatively high electric field is applied to the cells to fuse them together. If a pair of wire electrodes are used, the following problems are encountered: (1) The strength of the electric field between the electrodes is not uniform, so that it is difficult to accurately know the effective strength of the electric field applied to the cells. (2) The fused cells adhere to the electrodes, so that when they are separated from the electrodes, they are damaged or broken. (3) Multi-fusion occurs, that is, three or more cells are linked like a pearl chain. It is also known to use a pair of parallel plate electrodes to which a DC voltage is applied for fusing the cells that fill the space between the electrodes. Beside the above problems (2) and (3) there is a third problem that the degree of membrane contact between adjacent cells is low and consequently the rate of fusion is low. The cells to be fused are suspended in a solution, and the suspension is put in a space between a pair of opposed electrodes in the fusion chamber. If the amount of the suspension is relatively small such as, for example, 0.05 ml, small electrodes suffice and the fusion chamber enclosing the electrodes can be of a simple shape and construction so that it is relatively easy to handle the chamber. If the amount of the cell suspension, however, is relatively great such as, for example, 1 ml, the space between the electrodes and/or the length of the electrodes must be increased to accommodate the suspension. If the space between the electrodes is increased, the voltage applied to the electrodes must be increased, with resulting increase in the size of the instrument as a whole. If the length of the electrodes is increased, in order for a fusion chamber of a limited size to accommodate the elongated electrodes, the electrodes must be of a complicated shape. If long linear electrodes are used, the fusion chamber must be elongated, with resulting decrease in the easiness of manipulation of the instrument. The pages 165-182 of "The Journal of Membrane Biology 67 (1982)", refers to a basic article on the possiblities of cell-to-cell fusion induced by electric fields. Different ways or set-ups for achieving different types of cell fusion are shown, beginning with the fusion of a plurality of cells up to the fusion of only two cells. Furthermore, document EP-A-0 205 179 describes the use of a chamber for treating cells in an electric field, wherein two electrodes are basically formed in a cylindrical or ball-shaped manner so that, in case a voltage is applied, a field as uniform as possible is formed between them, whereby all the cells are subjected to the same electrical field intensity. In this context one of the electrodes is hollow and, separated by an insulating member, encloses the second electrode whose diameter is such that the distance between the inner surface of the first electrode and the outer surface of the second electrode is small. According to the invention, a fusion chamber for fusing cells is provided, including a pair of electrodes having opposite surfaces positioned at a predetermined distance apart from each other to allow the applying of a uniform electric field to a cell suspension therebetween, characterized in that, said fusion chamber is provided with at least one additional cavity for containing a different liquid and a removable lid for airtightly sealing said fusion chamber and said cavity. Hence, by means of the airtight sealing of the chamber receiving the cell suspension and by at least one additional chamber for receiving a further, different liquid, a cell fusion can be observed also for a longer period of time, for instance under the microscope, without the cell suspension losing its amount of liquid and accompanying cell destruction. Advantageous further developments result from the subclaims. The invention will be described in detail with reference to the accompanying drawing. Fig. 1 is a schematic diagram of an apparatus for fusing cells; Fig. 2 is a vertical section taken along line II - II of a fusion chamber shown in Fig. 1; Fig. 3 is a schematic top plan view of another form of a fusion chamber, with a lid taken off; Fig. 4 is a vertical section taken along line IV - IV of the fusion chamber in Fig. 3, with the lid tightly closing the fusion chamber; Fig. 5 is a schematic perspective view of a form of an fusion chamber according to the invention; Fig. 6 is a vertical section taken along line VI - VI of the fusion chamber shown in Fig. 5; Fig. 7 is a schematic top plan view of a modified form of the fusion chamber shown in Figs. 5 and 6; and Fig. 8 is a vertical section taken along line VIII-VIII of the fusion chamber shown in Fig. 7. It should be noted that in the drawing the relative dimensions of the component parts are not shown accurate but exaggerated for clarity of illustration, and that the dimensions of the parts should not be taken to limit the scope of the invention in any sense. Referring to Figs. 1 and 2, there is shown an apparatus for fusing cells which comprises a fusion chamber 10, and a source 12 of electricity and a controller 14. The fusion chamber 10 comprises a cavity 16 in which a pair of parallel plate electrodes 18a and 18b are provided and spaced apart from each other. The distance between the two electrodes 18a and 18b is determined in view of the source voltage so as to be as great as possible so long as an electric field strength necessary for effecting the intended cell fusion is obtained. The greater the height of the electrodes 18a and 18b is, the more cell suspension must be put between the electrodes. On the contrary, if the height of the electrodes is made too small, the nonuniform electric field caused adjacent the upper edges of the electrodes exerts an adverse influence on the cells. Therefore, the electrodes have such a height in view of the size of the cells as to prevent any nonuniform electric field from adversely affecting the process of cell fusion. The electrodes have a pair of terminals 20a and 20b for connection to the source 12. The cavity 16 has a bottom wall 16a between the two electrodes 18a and 18b. The bottom wall 16a is made of a material which provides a flat and smooth upper surface. A suspension 22 containing cells 24 to be fused together is put in the cavity 16 between the electrodes 18a and 18b. The source 12 comprises an AC source 26 and a DC source 28, which are connected to the terminals 20a and 20b through switches S₁ and S₂, respectively, so that an AC and a DC voltage are alternatively applied to the electrodes 18a and 18b in the fusion chamber 10. The cells 24 in the suspension 22 precipitate onto the bottom wall 16a of the cavity 16. By closing the switch S₁ an AC voltage having a predetermined frequency (within the range of 50 kHz to 5 MHz) is applied through the terminals 20a and 20b to the electrodes 18a and 18b thereby to generate an AC field therebetween, whereupon the cells 24 on the bottom wall surface 16a of the cavity 16 are lined in the direction of the electric field until adjacent cells are brought into close membrane contact with each other. Then the switch S₁ is opened and the switch S₂ is closed to apply a DC field to the cells to cause each adjacent pair of cells to be fused into a single cell. Since the electrodes 18a and 18b are parallel plate electrodes, the strength of the AC electric field applied to the cells is substantially uniform between the electrodes. In addition, since the AC field causes the cells on the bottom wall surface of the cavity 16 in the fusion chamber 10 to be lined perpendicularly to the electrodes 18a and 18b, that is, in the same direction as that of the electric field, the field strength conforms substantially to the calculated value, so that it is possible to exactly know the strength of the electric field applied to the cells. Since the bottom wall surface of the cavity 16 in the fusion chamber 10 is flat and smooth, the cells can move easily on the surface upon application of an AC field thereto. Since an AC field is applied to the cells that have precipitated onto the bottom wall surface of the cavity 16, the electrodes 18a and 18b can have a smaller height than otherwise, and in order to contain as much cell suspension as possible, the electrodes are spaced apart from each other as great a distance as possible. For a single pair of cells to be fused together the density or quantity of cells in the suspension must be adjusted preferably within a range of 10⁴/ml to 10⁵/ml. With such an apparatus, it is possible (1) to exactly know the effective electric field strength for cell fusion, (2) to bring cells into close membrane contact with each other by application of an AC field to them, thereby to increase the efficiency of fusion, (3) to cause cell fusion to be effected between the electrodes so that the fused cells can be taken out of the fusion chamber without any danger of damaging them, and (4) to obtain single pairs of fused cells. Turning to Figs. 3 and 4, wherein the same reference numerals as in Figs. 1 and 2 designate corresponding component parts, there is shown a fusion chamber 10 comprising a shallow container 32 which is circular in transverse section and open upward, and a lid or cover 34 for airtightly closing the container. A pair of hollow cylindrical electrodes 36a and 36b are concentrically arranged in a spaced apart relation to each other in the container and have an inner and an outer circumferential surface 38a and 38b, respectively, so as to define an annular space 40 therebetween. The electrodes 36a and 36b have a pair of terminals 20a and 20b which appear outside the container 32. As in the embodiment of Figs.1 and 2, the electrodes have such a height or axial length in view of the cells to be fused as to prevent any nonuniform electric field from exerting an adverse influence on formation of pairs of cells. A cell suspension 22 is put in the annular space 40, and the lid or cover 34 tightly closes the container 32. First, an AC field is generated between the electrodes 36a and 36b to form pairs of cells in each of which two cells closely contact each other, and then a DC pulse is applied to the cells to cause fusion of each pair of cells. The maximum quantity of the cell suspension 22 that fills the annular space 40 between the electrodes 36a and 36b is given as ℓ×d×h, that is, the product of the length ℓ (= 2πr₃) of the annular space 40, the distance (d) between the electrodes and the depth (h) of the space. The field strength E(V/cm) of a pair of parallel electrodes is given as E V₀/d where V₀ is the voltage impressed across the two electrodes, and d is the distance between the two electrodes. The field strength E is uniform. In the hollow cylindrical electrodes 36a and 36b, the field strengths E₁ and E₂ adjacent to the outer electrode 36a and the inner electrode 36b are given respectively as follows: E, = K/r₁, and E₂ = K/r₂ where K is a constant, r₁ is the radius of the inner circumference of the outer electrode 36a and r₂ is the radius of the outer circumference 38b of the inner electrode 36b. Therefore, E₁/E₂ = r₂/r₁ . If r₁ ≈ r₂, E₁ ≈ E₂, so that the strength of the electric field produced between the two electrodes 36a and 36b becomes approximately the same as that of the electric field produced between two parallel electrodes. The difference between the radii r₁ and r₂ is very small as compared with the radii. For example, r₁ is 50 mm and r² is 52 mm, so that the difference, that is, the above-mentioned distance d is 2 mm. Since the hollow cylindrical electrodes 36a and 36b have their respective radii approximating each other, they have the following advantages: (1) They have substantially the same electrical characteristics as parallel electrodes and enables cell fusion with substantially the same efficiency of fusion. (2) The annular shape of the electrodes and concentric arrangement thereof enable simplification of the structure of the fusion chamber, particularly, that of the electrodes. (3) The chamber can be made compact in size, so that it can be handled with ease on a microscope or in a clean bench. Now turning to Figs. 5 and 6, there is shown a fusion chamber 10 according to the invention which fusion chamber comprises a container 32 open upward and a lid or cover 34 for airtightly closing the container 32. Both the container 32 and the lid 34 are square in transverse section. The container 32 is formed in the inner bottom surface thereof with a central groove or cavity 42 and a pair of side grooves or cavities 44a and 44b at the outer opposite sides of the central cavity 42. A pair of spaced parallel plate electrodes 18a and 18b are provided in the central cavity 42. As in the previous embodiments, the electrodes are of such a height as to prevent any nonuniform electric field from adversely affecting formation of pairs of cells. A cell suspension 22 is put in the central cavity 42 while a different liquid 46 such as for example water is put in the side cavities 44a and 44b. When the lid 34 is placed on the container 32, the downwardly facing inner surface 36 of the lid 34 closely contacts the upper edge surfaces of the side walls 32a and 32b of the container 32 thereby to airtightly enclose the cell suspension 32 in the cavity 42 and the water in the cavities 44a and 44b within the same space in the fusion chamber 10. Under the condition, an AC voltage is applied to the electrodes 18a and 18b through the terminals 20a and 20b to generate an AC field therebetween thereby to form pairs of cells, whereupon a DC pulse is applied to the pairs of cells to fuse each pair into a single cell. During the process the temperature of the cell suspension 22 is raised due to the ambient temperature and the application of an AC field thereto so that the liquid component of the suspension 22 begins to evaporate. However, the lid 34 prevents the vapor from emerging out of the chamber 10, so that the vapor pressure rises to prevent further evaporation of the suspension 22. At the same time, evaporation of the water 46 in the side cavities 44a and 44b helps prevent evaporation of the suspension 22. Since evaporation of the cell suspension 22 is suppressed to the minimum, it is possible for a process of cell fusion to continue for a longer time than otherwise without adversely affecting the cells in the suspension. Figs. 7 and 8 show a modified form of the fusion chamber according to the invention shown in Figs, 5 and 6. The same reference numerals as in Figs. 5 and 6 designate corresponding parts. The container 32 is circular and formed with a central rectangular cavity 42 and a pair of circular pits 48a and 48b at the opposite sides of the cavity 42. A pair of parallel plate electrodes 18a and 18b are provided in the cavity 42, with a pair of terminals 20a and 20b for connection to an AC and a DC voltage source, not shown. In any of the above-mentioned embodiments of the invention, the bottom wall of the fusion chamber may be made of a transparent material at least between the electrodes so that the process of cell fusion with an electric field being applied to the cells can be observed through the transparent bottom wall with an inverted microscope. In this case the lid must be of such a material as to pass light for illumination.

Claims: A fusion chamber ( 10 ) for fusing cells ( 24 ) including a pair of electrodes having opposite surfaces positioned at a predetermined distance apart from each other to allow the applying of a uniform electric field to a cell suspension ( 22 ) therebetween, characterized in that , said fusion chamber ( 10 ) is provided with at least one additional cavity for containing a different liquid ( 46 ) and a removable lid ( 34 ) for airtightly sealing said fusion chamber ( 10 ) and said cavity. The fusion chamber of claim l, characterized in that , said pair of electrodes are parallel plate electrodes ( 18a , 18b ). The fusion chamber of claim 1, characterized in that , said pair of electrodes are cylindrical electrodes ( 36a , 36b ), wherein the first electrode ( 36a ) has an inner circumferential surface of a first radius ( r1 ), the second electrode ( 36b ) has an outer circumferential surface of a second radius ( r2 ) smaller than said first radius ( r1 ) and said second electrode is concentrically disposed in said first electrode ( 36a ). The fusion chamber of any one of the preceding claims, characterized in that , the floor of said fusion chamber ( 10 ) has a flat and smooth surface between said electrodes. The fusion chamber of any one of the preceding claims, characterized in that , the floor of said fusion chamber ( 10 ) is transparent at least between said electrodes. The fusion chamber of any one of the preceding claims, characterized in that , said removable lid ( 34 ) for airtightly sealing said fusion chamber ( 10 ) and said cavity is transparent. The fusion chamber of any one of the preceding claims, characterized by means ( 26 ) for producing an alternating current voltage. The fusion chamber of any one of the claims 1 to 6, characterized by means ( 28 ) for producing a direct current voltage. The fusion chamber of any one of the claims 1 to 6, characterized by means ( 12 ) for producing an alternating current voltage and a direct current voltage. The fusion chamber of claim 9, characterized by means ( S1, S2, 14 ) for effecting selective application of said AC and DC voltage to said electrodes.

Inventors: IMAI, KATSUYUKI, KOGA, MAMORU, MOCHIZUKI, TAKANORI, TAKAYAMA, SHINITIRO, TODA, KENZO, TOGAWA, YOSHIYUKI
Applicants: SHIMADZU CORPORATION
Application year: 1986
Main classification: C12M3
IPC codes: C12N13, C12M3, C12N1