Abstract:
The invention relates to a novel circuit arrangement for electrotransfection or electrofusion, which enables the transportation of DNA and/or other biologically active molecules to the nucleus of higher eukaryotic cells or the fusion of cells, independent of cell division and with reduced cell mortality.

Description:
The invention relates to a circuit arrangement for introducing nucleic acids, peptides, proteins and/or other biologically active molecules into the cell nucleus of eukaryotic cells by means of electric current, or for the treatment of cells, cell derivatives, subcellular particles and/or vesicles with electric current, consisting of at least two storage devices for quantities of electric charge, each supplied by a high-voltage power supply which each have at least one power semiconductor for transferring the quantities of charge present in the storage devices into a suspension in a cuvette and at least one monitoring device for controlling the power semiconductor. 
     BACKGROUND OF THE INVENTION 
     Since the place of action of eukaryotic DNA is the cell nucleus, DNA supplied from outside must enter the nucleus in order to be read out. Conventional transfection methods only bring about transport of DNA through the cell membrane into the cytoplasm. It is only because the nuclear membrane is temporarily dissolved during the cell division of higher eukaryotes that the DNA can passively enter the nucleus so that proteins encoded by it can be expressed. Only very small DNA molecules (oligonucleotides) can diffuse freely through the pores of the nuclear membrane. For the effective transfection of quiescent or weakly dividing cells it is thus necessary to create conditions which have the result that larger DNA molecules enter the nucleus through the nuclear membrane in sufficient quantity. The circuit arrangement described here makes this possible in higher eukaryotic cells. 
     STATE OF THE ART 
     It has been known for some time that DNA from a buffer can be introduced into cells with the aid of electric current. However, the circuit arrangements for electroporation described so far are based on the transport of DNA into the cytoplasm of higher eukaryotic cells so that the expression of transfected DNA remains dependent on the dissolution of the nuclear membrane during the cell division. None of the circuit arrangements for electroporation known so far is concerned with bringing DNA electrically specifically into the nucleus of higher eukaryotic cells. Thus, a circuit arrangement for electrotransfection optimised for electrical nucleus transport is not known. 
     U.S. Pat. No. 4,750,100 from Bio-Rad Laboratories, Richmond, USA (1986), describes a specific equipment structure which can provide a maximum of 3000 V at a maximum of 125 A by capacitor discharge. 
     U.S. Pat. No. 5,869,326 (Genetronics, Inc., San Diego, USA, 1996) describes a specific equipment structure by which means two, three or a plurality of pulses can be generated using two separate current sources. However it is not claimed or shown that these pulses have an effect which goes beyond the transport of DNA into the cytoplasm. 
     U.S. Pat. No. 6,008,038 and the European Patent Application EP 0 866 123 A1 (Eppendorf-Netheler-Hinz GmbH, Hamburg, 1998) describe a device with which short pulses of 10-500 μs and a maximum of 1.5 kV can be generated but again give no indication that certain conditions could lead to conveying DNA into the nucleus. 
     None of the circuit arrangements known so far is optimised to make it possible for DNA and/or other biologically active molecules to be effectively transported into the cell nucleus with low cell mortality. 
     The object of the invention is to provide a circuit arrangement which makes it possible for DNA and/or biologically active molecules to be transported effectively into the cell nucleus with low cell mortality. 
     DESCRIPTION OF THE INVENTION 
     In order to solve the object according to the invention it is provided that the first storage device is charged with the preset voltage (U 1 ) as a parameter and the second storage device is charged with a voltage U 2 =R×I 2 ×K 2 , wherein R is the resistance of the cuvette and the suspension contained therein, I 2  is the desired current and K 2  is a correction value which takes into account the cuvette properties and wherein at least one first pulse with the capacitor voltage (U 1 ) of the storage device can be transferred to the cell for a preset time (T 1 ) by controlling a power semiconductor. 
     In a development of the invention it is provided that without interruption at least one second pulse with the capacitor voltage (U 2 ) of the storage device can also be applied to the cuvette by controlling a power semiconductor, wherein the delivered quantity of charge in at least one selectable time interval can be measured by the monitoring device, wherein the preset desired quantity of charge is compared with the actual delivered quantity of charge and on reaching or exceeding the desired quantity of charge, the power semiconductor is blocked. 
     In addition to the possibility of determining the delivered quantity of charge using the current flowing from the storage device, alternatively the preset desired quantity of charge is compared with the actual delivered quantity of charge in an interval of time and on reaching or exceeding the desired quantity of charge, the power semiconductor is blocked. On this occasion, depending on the pulse shape used and the number of pulses, the time interval which can be selected for the determination can be individually predefined in order, for example, to determine the delivered quantity of charge during the first or each subsequent pulse. The delivered quantity of charge can be determined by determining the difference between the original charge at least of one of the storage devices and the residual charge. In this case it is possible that according to the number of pulses used, more than one of the at least two storage devices is used in a circuit fashion wherein each storage device is assigned at least one high-voltage power supply, a monitoring device and a power semiconductor to transfer the quantity of charge to the cuvette containing the cell suspension. For the pulse transfer it is provided that the first power semiconductor transfers a pulse of 2-10 kV/cm having a duration of 10-100 μs and a current density of at least 2 A·cm 2  and, without interruption, the second power semiconductor transfers a pulse having a current density of 2-14 A·cm 2  and a maximum duration of 100 ms. The time interval for determining the delivered quantity of charge can consequently be specified with the delivery of a first and/or preferably a second or each further pulse. 
     The delivered quantity of charge of the second pulse is preferably monitored wherein the switch-on time (T 2 ) of the second pulse can be specified by comparing the desired quantity of charge with the actual quantity of charge delivered by the measurement time and ends when the desired quantity of charge is reached and wherein a measurement cycle of 1 msec is provided to determine the actual quantity of charge, wherein during the time (T 2 ) the capacitor voltage decreases exponentially and the power semiconductor can be blocked on reaching the specified quantity of charge (Q 2 ). 
     Alternatively it is possible that after at least one predetermined time interval after triggering a first and/or second pulse, the flowing current is measured and if this exceeds or falls below a desired value, the pulse duration can be re-adjusted in order to keep the delivered quantity of charge constant. In another alternative it is possible that after at least one predetermined time interval after triggering a first and/or second pulse, the flowing current is measured and if this exceeds or falls below a desired value, an error message is generated to give a warning to the user of the device. It is furthermore possible that after at least one predetermined time interval after triggering a first and/or second pulse, the flowing current is measured and if this exceeds or falls below a desired value, the desired value is readjusted. 
     In order to determine any necessary constants, especially of the cuvette used with the cell suspension, it can be provided that a preliminary measurement of the resistance of the cuvette with the cell suspension is made. The other necessary pulse parameters are preferably pre-selected manually or if necessary specified by entering a code. It is thus also possible to use retrievable data via a card reader. The card reader can also be used at the same time to store the time profile of the voltage applied to the cuvette or the current flowing through the cuvette for documentation purposes for one or a plurality of pulse delivery processes on a commercially available memory card. This memory card is preferably used at the same time for storing the pulse parameters to be set. 
     As a result of the circuit regulation of the pulse delivery, the transfer of the envisaged quantity of charge is thus monitored in a reliable and advantageous fashion at least for one pulse and a controlled and sample-dependent transfer of a preset quantity of charge as well as a controlled monitoring to avoid any damage to the cells located in the sample can be achieved. 
     For further safety of the user and the samples used it is provided that an overcurrent cutoff is provided for the first and each subsequent pulse. The overcurrent cutoff thus allows the high-voltage pulse to be interrupted at any time in the event that preset limiting values are exceeded. 
     The high-voltage pulse of 2-10 kV/cm described is suitable for creating conditions such that DNA can enter the cell nucleus independently of the cell division. In order to keep cell damage low, this pulse is limited to between 10 and a maximum of 200 μs, preferably 10-50 μs. This is sufficient to achieve transfection independent of cell division. For example, such a short single high-voltage pulse was found to be optimum for the transfection of endothelial cells from the human umbilical vein. Another current pulse of lower field intensity or lower current strength or current density but of longer duration, following without interruption influences the efficiency of the transfection. As a result of the significantly lower current density, this pulse can persist significantly longer with little cell damage. An optimum current density or duration of the second pulse is obtained depending on the cell type and sensitivity of the cell. Such combined pulses are found to be optimal, for example, for primary human dermal fibroblasts or melanocytes or various human blood cells. In experiments using different cell lines and expression systems, the following was shown: the higher the current density of the second pulse, the stronger its influence on the transfection rate, i.e. the percentage of transfected cells. The lower the current density, the more the second pulse causes pure DNA transport into cells already transfected by the first pulse. The expression level of the transfected cells increases with increasing pulse duration but not the fraction of transfected cells. In order to maintain a precise cell-specific control of the transfection rate, the expression level and the cell vitality, the pulse duration and current density of the second pulse must therefore be controlled. 
     In order to achieve precise control of the pulse actually delivered to the cell suspension, in a preferred embodiment the delivered quantity of charge is controlled. In order to control the current strength or current density by a selectable capacitor voltage of the storage unit, the resistance of the cuvette and the cell suspension contained therein must be predefined initially. It was found that the resistance of the cuvettes when using aluminium electrodes varies during the pulse as a result of electrochemical processes. This variation is taken into account by a pulse-specific predefined correction value. Thus, precise pulse shapes for the second pulse can be predetermined using U 2 =R×I 2 ×K 2  by controlling the charge, where U 2  is the capacitor voltage with which the storage device is charged, R is the resistance of the cuvette and the cell suspension contained therein, I 2  is the desired current and K 2  is the pulse-specific correction value. 
     In one embodiment of the invention the ohmic cuvette resistance R can be measured directly before the beginning of pulse delivery by applying a test voltage and taken into account accordingly in the calculation of the voltage U 2 . Since the resistance measured before pulse delivery is subjected to larger fluctuations than the resistance during pulse delivery, presumably as a result of electrochemical processes, it is found to be advantageous to fixedly predefine the resistance R to calculate the capacitor voltage U 2  as a parameter. In a preferred embodiment of the invention the resistance of the cuvette is measured before the commencement of pulse delivery regardless of this in order to determine whether this lies within a predefined resistance window. If the measured resistance lies outside this window, there is a fault and the pulse delivery is not released. 
     For every cell type optimum conditions can be established for transfection rate, transfection intensity and cell vitality. In a preferred embodiment of the circuit arrangement the field intensity and duration of the first pulse and initial current intensity or current density and empirical duration of the second pulse can be selected and optimum conditions can simply be established for various cell types via a code. 
     The circuit arrangement can be used in an advantageous fashion for the transfection of quiescent or dividing eukaryotic cells. In the same way the circuit arrangement is also suitable for the transfection of primary cells such as human blood cells, pluripotent precursor cells from human blood, primary human fibroblasts, endothelial cells, muscle cells or melanocytes and can be used for diagnostic purposes or for the manufacture of a medicinal product for ex-vivo gene therapy. 
     The circuit arrangement according to the invention is furthermore also suitable, for example, for electrofusion, i.e., methods for the fusion of cells, cell derivatives, subcellular particles and/or vesicles by means of electric current, wherein, for examples the cells, cell derivatives, subcellular particles and/or vesicles are initially suspended in a suitable density in an aqueous solution, the suspension is then transferred to a cuvette and finally an electric voltage is applied to the electrodes of the cuvette and a current flow is generated through the suspension. Alternatively, for example, adherent cells, cell derivatives, subcellular particles and/or vesicles or however, also adherent cells with suspended cells, cell derivatives, subcellular particles or vesicles can be fused. 
     The circuit arrangement described here generates very high field intensities of 2 to 10 kV/cm which have the effect that DNA and/or biologically active molecules can enter the nucleus independently of the cell division. These field intensities are far above those normally used for electroporation and far beyond those sufficient for efficient opening of the pores in the cell membrane (on average 1 kV/cm according to Lurquin, 1997, Mol. Biotechnol. 7, 5). 
     The subject matter of the invention is thus a circuit arrangement for implementing a method for introducing nucleic acids, peptides, proteins and/or other biologically active molecules into the cell nucleus of higher eukaryotic cells using electric current wherein the introduction into the nucleus is achieved by a pulse having a field intensity 2-10 times that sufficient for opening the pores in the cell membrane and a duration of at least 10 μs and a current density of at least 2 A·cm 2 . 
     The introduction of nucleic acids, peptides, proteins and/or other biologically active molecules into the cell nucleus can be achieved by a pulse of 2-10 kV/cm, preferably 3-8 kV/cm, wherein the pulse is a maximum of 200 μs long. 
     The circuit arrangement is designed so that the first pulse can be followed without interruption by a current flow having a current density of 2 A·cm −2  up to a maximum of 14 A·cm 2 , preferably up to 5 A·cm 2 , of 1 ms up to a maximum of 100 ms, preferably up to 50 ms in length. 
     Since the circuit arrangement makes transfection possible regardless of the cell division, in addition to dividing cells, quiescent or weakly dividing primary cells can also be transfected. 
     In other preferred embodiments the higher eukaryotic cells comprise primary human fibroblasts, endothelial cells and melanocytes. 
     The eukaryotic cells transfected using the circuit arrangement according to the invention can also be used for diagnostic and analytic purposes to produce a pharmaceutical product for ex-vivo gene therapy. 
     The circuit arrangement according to the invention makes it possible to achieve transfection independent of cell division and thus to considerably speed up transfection experiments. In transfection experiments using expression vectors, an analysis according to promoter and expressed protein can be made even a few hours after the transfection. 
     The concept “biologically active molecules” means peptides, proteins, polysaccharides, lipids or combinations or derivatives of these molecules as long as they develop a biological affinity in the cell. 
     Electroporation buffers having a high ionic strength and high buffer capacity are especially suitable for use with the circuit arrangement according to the invention. 
     The following protocol can be used to introduce nucleic acids into the cell nucleus of eukaryotic cells: 1×10 5 -1×10 7  cells and up to 10 μg DNA are incubated in 100 μl electroporation buffer in a cuvette having a 2 mm interelectrode gap for 10 min at room temperature and then transfected according to the conditions according to the invention. Immediately afterwards the cells are washed out of the cuvette with 400 μl of cell culture medium and incubated for 10 min at 37° C. The cells are then plated out in 37° C. warm cell culture medium. 
     Suitable cuvettes are commercially available cuvettes for the electroporation of prokaryotes having an interelectrode gap or 2 mm or 1 mm, for example. 
     Evidence that the nucleic acids enter the cell nucleus independently of cell division can be furnished by analysing the cells which have not divided between transfection and analysis. This is achieved on the one hand by the transfection of non-dividing cells, such as for example cells of peripheral human blood and on the other hand for dividing cells by an analysis a few hours after transfection at a time when at most a fraction of the cells can have divided 
     The following abbreviations are used in addition to those in general use: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 FACS 
                 Fluorescence activated cell sorting 
               
               
                   
                 FCS 
                 Foetal calf serum 
               
               
                   
                 PBMC 
                 Peripheral blood mononuclear cells 
               
               
                   
                 PE 
                 Phycoerythrin 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLES 
     The following examples illustrate the invention but should not be regarded as restrictive. 
     Example 1 
     Transfection of Cytotoxic T Cells From Human Blood 
     Freshly prepared unstimulated (non-dividing) mononuclear cells from peripheral human blood (PBMC) were transfected with a vector which codes for the heavy chain of the mouse MHC class 1 protein H-2K k . 5×10 6  cells together with 5 μg of vector DNA in a buffer having a high buffer capacity (48 mM×pH −1 ) and high ionic strength (280 mM) were placed at room temperature in a cuvette having a 2 mm interelectrode gap and transfected by a 1000 V pulse of 100 μs duration, followed by a current flow having a current density of 5 A·cm −2  and 40 ms. Immediately afterwards, the cells were washed from the cuvette using 400 μl of culture medium, incubated for 10 minutes at 37° C. and then transferred to a culture dish with pre-heated medium. After incubating for 24 h, the cells were successively incubated with digoxigenin-coupled anti-H-2K k -antibody and Cy5-coupled anti-digoxigenin-antibody, as well as with a PerCP-coupled anti-CD8-antibody to identify human cytotoxic T cells and analysed using a flow cytometer (FACScalibur, Becton Dickinson). The number of dead cells was determined by staining with propidium iodide. As shown in  FIG. 1 , 74.3% of the living cells express the H-2K k  antigen which corresponds to a very high transfection efficiency. 
     Example 2 
     Transfection of Human Haematopoeitic Stem Cells (CD34) 
     CD34-positive cells were pre-enriched from freshly prepared PBMC described as in Example 1 by magnetic cell sorting. Respectively 1×10 4  CD34-positive cells were then mixed with 1×10 6  PBMCs, placed together with 5 μg H-2K k -expression vector DNA in a buffer having a high buffer capacity (54 mM×pH −1 ) and high ionic strength (260 mM) at room temperature in a cuvette having a 2 mm interelectrode gap and transfected by a 1000 V pulse of 100 μs duration, followed by a current flow having a current density of 4 A·cm −2  and 20 ms duration. Immediately afterwards, the cells were washed from the cuvette using 400 μl of culture medium, incubated for 10 minutes at 37° C. and then transferred to a culture dish with pre-heated medium. After incubating for 16 h, the cells were successively incubated with phycoerythrin-coupled anti-H-2K k -antibody, as well as with an APC-coupled anti-CD34 antibody to identify human haematopoietic stem cells and analysed using a flow cytometer (FACScalibur, Becton Dickinson). The number of dead cells was determined by staining with propidium iodide. As shown in  FIG. 2 , 66.7% of the cells express the H-2K k  antigen which corresponds to a high transfection efficiency. 
     Example 3 
     Transfection of Human Neonatal Dermal Fibroblasts (NHDF-Neo) 
     Human neonatal dermal fibroblasts (5×10 5  cells) together with 5 μg H-2K k -expression vector DNA were placed in a buffer having a high buffer capacity (67 mM×pH −1 ) and high ionic strength (380 mM) at room temperature in a cuvette having a 2 mm interelectrode gap and transfected by a 1000 V pulse of 100 μs duration, followed by a current flow having a current density of 6 A·cm −2  and of 33 ms duration. Immediately afterwards, the cells were washed from the cuvette using 400 μl of culture medium, incubated for 10 minutes at 37° C. and then transferred to a culture dish with pre-heated medium. After incubating for 5 h, the cells were incubated with a Cy5-coupled anti-H-2K k -antibody and analysed using a flow cytometer (FACScalibur, Becton Dickinson). The number of dead cells was determined by staining with propidium iodide. As shown in  FIG. 3 , 93% of the cells express the H-2K k  antigen which corresponds to a very high transfection efficiency. 
     Example 4 
     Transfection of Human Neonatal Melanocytes 
     Human neonatal melanocytes (2.5×10 5  cells) together with 5 μg H-2K k -expression vector DNA were placed in a buffer having a high buffer capacity (54 mM×pH −1 ) and high ionic strength (260 mM) at room temperature in a cuvette having a 2 mm interelectrode gap and transfected by a 1000 V pulse of 100 μs duration, followed by a current flow having a current density of 6 A·cm −2  and 33 ms duration. Immediately afterwards, the cells were washed from the cuvette using 400 μl of culture medium, incubated for 10 minutes at 37° C. and then transferred to a culture dish with pre-heated medium. After incubating for 5 h, the cells were incubated with a Cy5-coupled anti-H-2K k -antibody and analysed using a flow cytometer (FACScalibur, Becton Dickinson). The number of dead cells was determined by staining with propidium iodide. As shown in  FIG. 4 , 75.1% of the cells express the H-2K k  antigen which corresponds to a very high transfection efficiency. 
     Example 5 
     Transfection of Human Endothelial Cells From the Umbilical Vein (HUVEC) 
     Endothelial cells from the human umbilical vein (1×10 6  cells) together with 5 μg H-2K k -expression vector DNA were placed in a buffer having a high buffer capacity (67 mM×pH −1 ) and high ionic strength (378 mM) at room temperature in a cuvette having a 2 mm interelectrode gap and transfected by a 1000 V pulse of 100 μs duration. Immediately afterwards, the cells were washed from the cuvette using 400 μl of culture medium, incubated for 10 minutes at 37° C. and then transferred to a culture dish with pre-heated medium. After incubating for 5 h, the cells were incubated with a Cy5-coupled anti-H-2K k -antibody and analysed using a flow cytometer (FACScalibur, Becton Dickinson). The number of dead cells was determined by staining with propidium iodide. As shown in  FIG. 5 , 49.7% of the cells express the H-2K k  antigen which corresponds to a high transfection efficiency. 
     Example 6 
     Transfection of the Human Cell Line K562 
     K562 cells (1×10 6  cells) together with 5 μg H-2K k -expression vector DNA were placed in a buffer having a high buffer capacity (24 mM×pH −1 ) and high ionic strength (254 mM) at room temperature in a cuvette having a 2 mm interelectrode gap and transfected by a 1000 V pulse of 100 μs duration, followed by a current flow having a current density of 8 A·cm −2  and 10 ms duration. Immediately afterwards, the cells were washed from the cuvette using 400 μl of culture medium, incubated for 10 minutes at 37° C. and then transferred to a culture dish with pre-heated medium. After incubating for 4 h, the cells were incubated with a Cy5-coupled anti-H-2K k -antibody and analysed using a flow cytometer (FACScalibur, Becton Dickinson). The number of dead cells was determined by staining with propidium iodide. As shown in  FIG. 6 , 69.5% of the cells express the H-2K k  antigen which corresponds to a very high transfection efficiency. 
     Example 7 
     Transfection Efficiency and Average Fluorescence Intensity of Cycle3-GFP-Transfected CHO Cells 
     In order to investigate the transfection efficiency and the average fluorescence intensity of transfected cells as a function of the quantity of charge flowing in the second pulse, respectively 7×10 5  CHO cells together with 5 μg Cycle3-GFP-vector-DNA were placed in electroporation buffer in a cuvette having an interelectrode gap of 2 mm and transfected by a 1000 V, 10 μs pulse and subsequent second pulses differing in the variation of the current intensity or current density and pulse time. After cultivation for 5 hours, the cells were analysed using a flow cytometer.  FIG. 7  shows the transfection efficiency determined as a function of the integral of the current over the pulse time (the quantity of charge Q). It is found that the transfection efficiency can be increased with increasing current intensity (open circles). An increase in the pulse time for the same current intensity on the other hand results in no appreciable increase in efficiency (closed circles). The fluorescence intensity (brightness) of the transfected cells increases with increasing quantity of charge Q, with saturation being reached for high Q. No major differences are found whether the increase in Q was achieved by increasing the current intensity (open circles) or increasing the pulse length (closed circles). 
     Example 8 
     Transfection Efficiency and Average Fluorescence Intensity of Cycle3-GFP-Transfected Jurkat Cells 
     In order to investigate the transfection efficiency and the average fluorescence intensity of transfected cells as a function of the quantity of charge flowing in the second pulse, respectively 4×10 5  Jurkat cells together with 5 μg Cycle3-GFP-vector-DNA were placed in electroporation buffer in a cuvette having an interelectrode gap of 2 mm and transfected by a 1000 V, 10 μs pulse and subsequent second pulses differing in the variation of the current intensity or current density and pulse time. After cultivation for 5 hours, the cells were analysed using a flow cytometer.  FIG. 8  shows the transfection efficiency determined as a function of the integral of the current over the pulse time (the quantity of charge Q). As when using CHO cells, it is found that the transfection efficiency can be increased with increasing current intensity (open circles). An increase in the pulse time for the same current intensity on the other hand results in no appreciable increase in efficiency (closed circles). The fluorescence intensity (brightness) of the transfected cells increases with increasing quantity of charge Q, with saturation being reached for high Q. No major differences are found whether the increase in Q was achieved by increasing the current intensity (open circles) or increasing the pulse length (closed circles). 
     Example 9 
     Transfection Efficiency and Average Fluorescence Intensity of H-2K k -Transfected Jurkat Cells 
     In order to investigate the transfection efficiency and the average fluorescence intensity of transfected cells as a function of the quantity of charge flowing in the second pulse, respectively 1×10 6  Jurkat cells together with 2 μg of H2K k -expression vector DNA were placed in electroporation buffer in a cuvette having an interelectrode gap of 2 mm and transfected by a 1000 V, 10 μs pulse and subsequent second pulses differing in the variation of the current intensity or current density and pulse time. After cultivation for 3.5 hours, the cells were incubated with Cy5-coupled anti-H2K k  and analysed using a flow cytometer.  FIG. 9  shows the transfection efficiency determined as a function of the integral of the current over the pulse time (the quantity of charge Q). It is found that the transfection efficiency can be increased with increasing current intensity (open circles). An increase in the pulse time for the same current intensity on the other hand results in no appreciable increase in efficiency (closed circles). The fluorescence intensity (brightness) of the transfected cells increases with increasing quantity of charge Q, with saturation being reached for high Q. No major differences are found whether the increase in Q was achieved by increasing the current intensity (open circles) or increasing the pulse length (closed circles). 
     The invention is explained further with reference to the following figures. 
    
    
     
       In the figures 
         FIG. 1  shows a transfection of cytotoxic T cells from human blood, 
         FIG. 2  shows a transfection of pluripotent precursor cells from human blood, 
         FIG. 3  shows a transfection of human neonatal dermal fibroblasts, 
         FIG. 4  shows a transfection of human neonatal dermal melanocytes, 
         FIG. 5  shows a transfection of human endothelial cells from the umbilical cord, 
         FIG. 6  shows a transfection of the cell line K562 (analysis 4 h after transfection), 
         FIG. 7  shows an investigation of the transfection efficiency as a function of the current intensity, pulse time and quantity of charge and of the expression intensity as a function of the quantity of charge, experiment using the CHO cell line, 
         FIG. 8  shows an investigation of the transfection efficiency as a function of the current intensity, pulse time and quantity of charge and of the expression intensity as a function of the quantity of charge, experiment using the Jurkat cell line, 
         FIG. 9  shows an investigation of the transfection efficiency as a function of the current intensity, pulse time and quantity of charge and of the expression intensity as a function of the quantity of charge, experiment using the Jurkat cell line and the surface marker protein H-2K k , 
         FIG. 10  shows a block diagram of an electroporator circuit, 
         FIG. 11  shows a circuit diagram of a control panel, 
         FIG. 12  shows a circuit diagram of a card reader, 
         FIG. 13  shows a circuit diagram of a supply unit, 
         FIG. 14  shows a circuit diagram of a HV power supply, 
         FIG. 15  shows a circuit diagram of an HV switch, 
         FIG. 16  shows a circuit diagram of a current regulating system, 
         FIG. 17  shows a circuit diagram of a flash recognition system, 
         FIG. 18  shows a circuit diagram of a control system, and 
         FIG. 19  is a flow diagram to explain the sequence of the pulse delivery processes. 
     
    
    
       FIG. 1  shows the flow cytometric analysis of PBMC which had been transfected with H-2K k -expression vector. The cells were successively incubated with digoxigenin-coupled anti-H-2K k -antibody and Cy5-coupled anti-digoxigenin-antibody, as well as with a PerCP-coupled anti-CD8-antibody to identify human cytotoxic T cells and were analysed using a flow cytometer (FACScalibur, Becton Dickinson). (FL- 2 , FL- 3 =fluorescence channel  2 ,  3 ; SSC=sideward scatter, FSC=forward scatter, PerCP=peridinin chlorophyll protein, CD=cluster of differentiation). 
       FIG. 2  shows the flow cytometric analysis of CD34-positive stem cells enriched from PBMC which had been transfected with H-2K k -expression vector. The cells were successively incubated with phycoerythin-coupled anti-H-2K k -antibody, as well as with a APC-coupled anti-CD34-antibody to identify human CD34 positive haematopoietic stem cells and were analysed using a flow cytometer (FACScalibur, Becton Dickinson). (FL- 1 , FL- 3 =fluorescence channel  1 ,  3 ; SSC=sideward scatter, FSC=forward scatter, PE=phycoerythrin, APC=allophycocyanin, CD=cluster of differentiation). 
       FIG. 3  shows the flow cytometric analysis of human neonatal dermal fibroblasts (NHDF-Neo), which had been transfected with H-2K k -expression vector. The cells were incubated with Cy5-coupled anti-H-2K k  and analysed using a flow cytometer (FACScalibur, Becton Dickinson). (FL- 1 , FL- 2 , FL- 3 =fluorescence channel  1 ,  2 ,  3 ; SSC=sideward scatter, FSC=forward scatter). 
       FIG. 4  shows the flow cytometric analysis of human neonatal melanocytes (NHEM-Neo), which had been transfected with H-2K k -expression vector. The cells were incubated with Cy5-coupled anti-H-2K k  and analysed using a flow cytometer (FACScalibur, Becton Dickinson). (FL- 1 , FL- 2 , FL- 3 =fluorescence channel  1 ,  2 ,  3 ; SSC=sideward scatter, FSC=forward scatter). 
       FIG. 5  shows the flow cytometric analysis of endothelial cells from human umbilical cord (HUVEC), which had been transfected with H-2K k -expression vector. The cells were incubated with Cy5-coupled anti-H-2K k  and analysed using a flow cytometer (FACScalibur, Becton Dickinson). (FL- 1 , FL- 2 , FL- 3 =fluorescence channel  1 ,  2 ,  3 ; SSC=sideward scatter, FSC=forward scatter). 
       FIG. 6  shows the flow cytometric analysis of the human cell line K562 which had been transfected with H-2K k -expression vector. The cells were incubated with Cy5-coupled anti-H-2K k  and analysed using a flow cytometer (FACScalibur, Becton Dickinson). (FL- 1 , FL- 2 , FL- 3 =fluorescence channel  1 ,  2 ,  3 ; SSC=sideward scatter, FSC=forward scatter). 
       FIG. 7  shows a graphical representation of the transfection efficiency of CHO cells and the average fluorescence intensity (brightness) of the positive cells as a function of the quantity of charge Q which has flowed. The CHO cells were transfected with Cycle3-GFP expression vector and analysed after five hours using a flow cytometer (FACScalibur, Becton Dickinson). Closed circles correspond to a gradual increase in the pulse time for the same current intensity (2 A) or current density (4 A·cm −2 ), open circles correspond to an increase in current intensity. 
       FIG. 8  shows a graphical representation of the transfection efficiency of Jurkat cells and the average fluorescence intensity (brightness) of the positive cells as a function of the quantity of charge Q which has flowed. The Jurkat cells were transfected with Cycle3-GFP expression vector and analysed after five hours using a flow cytometer (FACScalibur, Becton Dickinson). Closed circles correspond to a gradual increase in the pulse time for the same current intensity (2 A) or current density (4 A·cm −2 ), open circles correspond to an increase in current intensity. 
       FIG. 9  shows a graphical representation of the transfection efficiency of Jurkat cells and the average fluorescence intensity (brightness) of the positive cells as a function of the quantity of charge Q which has flowed. The Jurkat cells were transfected with H-2K k  expression vector and incubated after 3.5 hours with a Cy5-coupled anti-H-2K k  and analysed using a flow cytometer (FACScalibur, Becton Dickinson). Closed circles correspond to a gradual increase in the pulse time for the same current intensity (2 A) or current density (4 A·cm −2 ), open circles correspond to an increase in current intensity. 
       FIG. 10  shows a block diagram of the electroporator  1  with the necessary individual components. These comprise an adjusting unit  2 , a control unit  3  to which a voltage supply unit  4  is connected as well as at least two HV power supplies  5 ,  6  with following storage devices  7 ,  8  and two power semiconductors  9 ,  10  provided for pulse delivery. The power semiconductors  9 ,  10  are controlled via a potential divider stage  11 ,  12  by the control unit  3  by means of an HV switch  13  and a regulating unit  14 . The storage devices  7 ,  8  are directly connected to the inputs of the power semiconductors  9 ,  10 , wherein the storage devices  7 ,  8  can consist of one or a plurality of capacitors depending on the field strength and the pulse duration. The power semiconductor  9  can for example consist of an IGBT and the power semiconductor  10  can consist of a MOSFET. However, the term “power semiconductor” should comprise all other electronic components or component assemblies by which means the voltages and currents to be switched within the scope of the invention can be switched with the required switching times. The output of the IGBT is directly connected to the cuvette connection  15  whereas the output of the MOSFET  10  is connected via a resistance  16  and a diode  17  to the cuvette connection  15  so that no pulse can flow back via the second power semiconductor  10  if both power semiconductors  9 ,  10  are controlled simultaneously. For this purpose the diode  17  is connected to the cuvette connection  15  on the cathode side. The second cuvette connection  18  is connected to earth via a resistance  19 . The resistance  19  comprises a measuring shunt to measure the voltage drop and supply to an overcurrent switching stage  20 . The overcurrent switching stage  20  can interrupt the pulse delivery by means of a switch  21  via the potential divider stage  11  and the HV switch  13  whereas a second overcurrent switching stage  22  interrupts a control system of the regulating unit  14  for the MOSFET  10  via a switch  23 . The voltage applied via the resistance  16  is fed to the overcurrent switching stage  22  in order to bring about a current switchoff in the event that the maximum current is exceeded. Since the resistance  16  is located directly in the high-voltage circuit, the switch  23  is located after the potential divider stage  12  so that no high-voltage pulses can enter the control unit  3  and the operating staff are not endangered. In the case of the overcurrent switching stage  20 , the low-resistance measuring resistance  19  lies behind the cuvette connections  15 ,  18  and is connected to earth so that the transmission of high-voltage pulses can be eliminated. Depending on the intended usage of the electroporator  1 , one or a plurality of high-voltage power supplies  5 ,  6  with the relevant storage devices  7 ,  8  and the necessary potential divider stages  11 ,  12  and HV switch  13  or regulating unit  14  to control the power semiconductors  9 ,  10  can be used. The storage devices  7 ,  8  are equipped with one or a plurality of capacitors of the required capacity and breakdown voltage so that a suitably high quantity of charge can be stored and transferred to the cuvette connection  15 . 
     The following  FIGS. 11 to 18  shows the circuit diagrams of the individual components in the block diagram. 
       FIG. 11  shows a circuit diagram of the control panel for entering the parameter signals to be set wherein these can be preselected via a pushbutton switch  30  and checked visually using display elements  31 . LEDs  32  shows when the equipment is ready for operation. The necessary parameters are prepared in the circuit and transmitted via a connector  33  to the control system in accordance with  FIG. 14 . 
       FIG. 12  shows a circuit diagram of a card reader  34  via which preset parameters for certain biological substances are read in and transmitted to the control unit as shown in  FIG. 14 . 
       FIG. 13  shows a circuit diagram of the supply unit which substantially consists of a 150/230 Volt changeover switch  35 , a transformer stage  36  with primary-side wiring and voltage lead and secondary regulating stages to produce the necessary operating voltages. For this purpose a plurality of voltage regulators  38  are inserted after the rectifier  37 . 
       FIG. 14  shows a circuit diagram of the two HV power supplies  5 ,  6  which can be identified from the block diagram. Both HV power supplies  5 ,  6  are acted upon by the voltage U 1  from the supply unit, wherein each regulating stage  39  receives a control signal U3on, U5on from the control system and the applied voltage U 1  charges the storage device  7 ,  8  consisting of a plurality of capacitors, in pulsed mode via a transformer stage  40 . The desired voltage reached is transmitted via output signals U3sense, U5sense of the control system as shown in  FIG. 15 . The voltage U 5  of the storage device  7  is fed to an HV switch  13  as shown in  FIG. 15  and the voltage U 3  of the storage device  8  is fed to a current regulating stage as shown in  FIG. 16 . 
       FIG. 15  shows a circuit diagram of the HV switch  13 . The HV switch  13  receives the signal HIN generated by the pulse monitoring stage as shown in  FIG. 16  to control the first power semiconductor  9 . This transmits the applied voltage U 5  to a solder pad  41  for the HV cable for connecting the cuvette which is then connected to earth via a second solder pad  42  via a low-resistance measuring resistance. An overcurrent cutoff stage  20  delivers a control signal for the control unit as shown in  FIG. 18  for switching off the power semiconductor  9  in the event of a preset maximum current rise being exceeded. The first solder pad  41  is further connected to the voltage output U 4  of the current regulating stage from  FIG. 12  in order that a controlled current flow into the cuvette to deliver a specific quantity of charge can be achieved following the high-voltage pulse. The current regulating stage from  FIG. 16  receives the control signals from the control unit from  FIG. 18  via a potential divider stage and regulates the voltage U 3  applied to the storage device  8  to the voltage U 4  delivered via the solder pad  41 . In this case, according to the invention, Q regulation or current regulation is used whereby the charge in the storage device  8  is determined at predefined time intervals of, for example, one millisecond and the delivered quantity of charge is determined taking into account the original charge. 
       FIG. 18  shows a circuit diagram of the control unit  3  which either takes into account the preset manual values or the values entered via a card reader  34  and controls the current regulating unit  14  as shown in  FIG. 16  on the basis of further monitoring signals. The HV switch  13  as shown in  FIG. 15  however is controlled after manually triggering the high-voltage pulse via a pulse monitoring stage  43  as shown in  FIG. 17  so that after the HV pulse has been delivered, the quantity of charge can be monitored via the current regulating unit  14  as shown in  FIG. 16 . 
     The pulse parameters can thus on the one hand be preset manually and on the other hand via a card reader so that when a pulse is triggered manually via the existing regulating electronics, a high-voltage pulse with or without monitoring of the flowing current and if necessary, a continuous current signal with monitoring of the quantity of charge can be delivered via a second HV power supply. 
       FIG. 19  shows a schematic flow diagram of the operating sequence of a pulse delivery process controlled by the control unit  3  (see  FIG. 10 ) according to a preferred embodiment of the invention. First, the required pulse parameters are predefined manually or by reading out a memory card (not shown). After starting the process (e.g. by actuating a corresponding trigger button), the ohmic resistance of the cuvette is first measured by briefly applying a low voltage (e.g. 12 V) to the cuvette connections  15 ,  18  and a subsequent current measurement (e.g. for 2 ms) in step  44 . As part of the interrogation  45  it is checked whether this resistance lies within a predefined window. If not, the subsequent process is interrupted. The measured resistance is not used subsequently to calculate the charging voltage U 2  in the present embodiment of the invention. If the resistance lies in order within the predefined window, the storage devices  7 ,  8  are charged to the predefined voltages U 1  and U 2  in step  46 . When the desired charging voltages are achieved, the charging by the HV power supplies  5 ,  6  is switched off. During the following pulse delivery, no recharging of the storage devices takes place. The pulse delivery for the high-voltage pulse then begins in step  47  by closing the semiconductor switch  9 . As a result, a relatively high current flows through the cell. An excessively steep current rise is recognised by the overcurrent cutoff stage  20  and results in immediate opening of the switch  9  for safety reasons and interrupts the routine. In the present embodiment the high-voltage pulse is terminated after a predefined time of a few microseconds whereupon the second pulse follows immediately and without interruption. For this purpose in step  48  the second semiconductor switch  10  is already closed a short time before opening the first semiconductor switch  9  so that there is an interruption-free transition between the two pulses. In the short time interval in which both high-voltage switches  9 ,  10  are closed simultaneously, the diode  17  prevents any higher voltage from being able to flow from the storage device  7  into the storage device  8 . The semiconductor switch  10  then remains open (provided that the maximum current is not exceeded by an overcurrent cutoff stage  22 ) until the predefined charge Q has flowed through the cuvette. For this purpose in step  49  the current flowing through the cuvette is measured and integrated in predefined time intervals (e.g. 1 ms). As soon as the predefined charge has been reached (see interrogation  50 ), the switch  10  is opened and the routine is terminated. The capacity of the storage device  8  is selected so that the voltage decreases gradually or slowly during the duration of the second pulse. If as a result of a fault, the predefined desired charge is still not yet achieved even when the storage device is almost completely discharged, the process will also be interrupted after a suitably selected time limit has been exceeded. 
     REFERENCE LIST 
     
         
         
           
               1  Electroporator 
               2  Adjusting unit 
               3  Control unit 
               4  Voltage supply unit 
               5  HV power supply 
               6  HV power supply 
               7  Storage device 
               8  Storage device 
               9  Power semiconductor 
               10  Power semiconductor 
               11  Potential divider stage 
               12  Potential divider stage 
               13  HV switch 
               14  Regulating unit 
               15  Cuvette connection 
               16  Resistance 
               17  Diode 
               18  Cuvette connection 
               19  Resistance 
               20  Overcurrent cutoff stage 
               21  Switch 
               22  Overcurrent cutoff stage 
               23  Switch 
               30  Push-button switch 
               31  Display element 
               32  LED 
               33  Connector 
               34  Card reader 
               35  Changeover switch 
               36  Transformer 
               37  Rectifier 
               38  Voltage regulator 
               39  Regulating stage 
               40  Transformer stage 
               41  Solder pad 
               42  Solder pad 
               43  Pulse monitoring stage 
               44  to  51  steps