Abstract:
A method and an apparatus for reprogramming living cells without using viruses. In that method a cocktail comprising at least two transcription factors and a microRNA is transfected into the interior of at least one cell in order to convert this cell into iPS cells or into another type of cell, by storing the cells to be converted in an aqueous environment of the cocktail without viral carriers and focusing a femtosecond laser in a laser scanning microscope with a numerical aperture between 0.9 and 1.5 on a cell membrane of the cell to be reprogrammed and controlling the position of the focus. The exposure period and laser power for the optical treatment of the cell such that the focus depending on the pulse repetition frequency with an output between 7 mW and 100 mW generates a transient small-pore hole with a size up to 500 nm.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application No. DE 10 2015 101 838.1, filed Feb. 9, 2015 which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The invention is directed to a method and an apparatus for reprogramming living cells, particularly for virus-free reprogramming of adult and embryonic stem cells, iPS cells and already differentiated cells. 
       BACKGROUND OF THE INVENTION 
       [0003]    Medical use of human adult and embryonic stem cells entails a number of problems. Adult stem cells are difficult to isolate, often only possess the possibility of differentiating into specific tissue cells (multipotent cells) and are often damaged when extracted from patients. 
         [0004]    Embryonic human stem cells, however, are pluripotent. They can differentiate into any cell type and can renew themselves while retaining pluripotency. However, obtaining these cells requires the use of fertilized human ova. The possibilities for clinical use are limited on bioethical grounds. Further, immunosuppressive agents must be administered because of the risk of rejection in regenerative medicine when using a graft produced from human embryonic stem cells. 
         [0005]    The above-mentioned drawbacks in the clinical use of stem cells do not arise when using induced pluripotent stem cells (iPS). These iPS cells are generated by reprogramming already differentiated cells to form undifferentiated cells (reversion) which behave like the embryonic stem cells. The exact mechanism, however, is unknown. Human iPS cells were produced from human fibroblasts for the first time in 2007 (Takahashi et al.,  Cell  131 (2007) 861-872). 
         [0006]    There are various methods of reprogramming However, many methods such as cell nucleus transfer or cell transfusion again require the use of ova. 
         [0007]    Direct programming, in which typically three or four specific transcription factors or microRNA are transfected into differentiated cells, does not have this disadvantage. It has been assumed heretofore that the transcription factors must remain stable for at least one week. For this reason, they have been integrated and applied in genomic DNA of a retrovirus or lentivirus (Miyazaki et al.,  Jpn. J. Clin. Oncol.  42 (2012) 773-779). These pioneers of viral reprogramming of cells were awarded the Nobel prize for medicine and physiology in 2012. 
         [0008]    Aside from the possibility of first producing iPS cells through reprogramming and subsequently selectively differentiating them for medical use, there is also the possibility of partial reprogramming In this case, no pluripotent iPS cells are generated, but rather either the required differentiated cells are generated directly, e.g., a heart cell from a skin cell, or multipotent cells are generated which can only differentiate into specific cells (Efe et al.,  Nature Cell Biology  13 (2011) 215-222; Vierbuchen et al.,  Nature  463 (2010) 7284). Viruses are also commonly used in this case, but three transcription factors are often sufficient. With partial reprogramming, the hope is also to avoid uncontrolled effects such as the generation of cancerous tissue and unpredictable cell mutations. 
         [0009]    To date, clinical use of human reprogrammed cells has been limited or rendered impossible by the use of viruses. There is the further disadvantage of low conversion efficiency, i.e., too few vital cells in proportion to lethal cells after viral injection transcription. 
         [0010]    A further disadvantage consists in that the method for producing iPS cells in the form of three-dimensional clusters (iPS colony) is cumbersome and currently takes at least one week. 
         [0011]    Required grafts are produced from the iPS cells or partially reprogrammed cells in complicated steps under in vitro conditions. In addition, iPS cells could be deposited directly into a target tissue by injection. To date, only embryonic and adult stem cells have been injected directly into the damaged target tissue (e.g., into the heart muscle, spinal cord). It is hoped in this way to obtain efficient conversions into the required differentiated cell type, since the environment is already made up of these cells and suitable differentiation factors are available. A reprogramming of cells within tissue is currently impossible. Further, in-situ reprogramming within the tissue to be treated in the patient would be of especially great interest. 
         [0012]    Apart from its application in regenerative medicine, reprogramming of cancer stem cells in the patient&#39;s body in particular would be of great interest. 
         [0013]    Accordingly, the following problem areas exist: 
         [0014]    a) the use of viruses limits clinical use, 
         [0015]    b) the efficiency of obtaining reprogrammed cells is low, 
         [0016]    c) the production of reprogrammed cells is generally cumbersome, 
         [0017]    d) the production of reprogrammed cells in a tissue environment is impossible, 
         [0018]    e) the reprogramming of cells in diseased target tissue in a patient by viral methods is impossible. 
         [0019]    Various methods are currently being researched which would allow direct reprogramming without the use of viruses. Accordingly, a reprogramming of mouse skin cells was implemented by the so-called STAP method (stimulus-triggered acquisition of pluripotency) by a Japanese research team based on variation of the microenvironment as published in the prestigious journal  Nature  (Obokata et al.,  Nature  505 (2014) 641-647). However, the method has not yet been corroborated by other research groups. To date, it has only been applied in animal cells. Moreover, a large number of cells die because the microenvironment has been modified to transient extreme pH gradients. A reprogramming of cells in a tissue environment is not provided with this method. 
         [0020]    Heretofore, the reprogramming of cells has usually been carried out using viruses which hinder clinical application. To date, the new STAP method is limited to animal cells and entails a high mortality rate. Additionally, all reprogramming has so far been carried out with a low efficiency and cumbersome, time-consuming preparatory steps. To date, direct reprogramming in an in vitro tissue or directly in vivo in the patient has been impossible. 
       SUMMARY OF THE INVENTION 
       [0021]    Therefore, it is the object of the invention to find a novel possibility for direct, efficient and fast cell reprogramming without using viruses and with the option of direct reprogramming in vital tissue. 
         [0022]    According to the invention, this object is met by a method for reprogramming living cells in which a cocktail comprising at least two transcription factors and a microRNA is transfected into the interior of at least one cell in order to convert this cell into iPS cells or into another type of cell, having the following steps: 
         [0023]    preparation of the cocktail without viral carriers in an aqueous environment of the at least one cell to be reprogrammed, 
         [0024]    providing a radiation source in the form of a femtosecond laser with a pulse repetition frequency ranging between 50 MHz and 2 GHz with a wavelength in the range of from 700 to 1200 nm, 
         [0025]    focusing a laser beam of the femtosecond laser by means of a laser scanning microscope with a numerical aperture of between 0.9 and 1.5 on a cell membrane of a selected cell in a sample with the at least one cell on a displaceable x-y table, 
         [0026]    directing an attenuated, nondestructive laser beam of the femtosecond laser, which is also used for optical treatment, to the device and observing the at least one selected cell in the focus of the laser scanning microscope by means of a scanner, and 
         [0027]    controlling the position of the at least one cell in the focus, the exposure period and laser power for the optical treatment such that the focus depending on the pulse repetition frequency with an output between 7 mW and 100 mW generates a transient small-pore hole with a size in the range of up to 500 nm within the cell membrane of the cell in order to allow a diffusion of the cocktail for multiple reprogramming of the cell through the cell membrane into the interior of the cell such that a virus-free optical multiple reprogramming takes place. 
         [0028]    The irradiation for reprogramming by means of a femtosecond laser is preferably carried out at a frequency between 75 and 85 MHz and with a center wavelength between 700 and 900 nm by means of the laser scanning microscope via a microscope objective with a high numerical aperture between 1.1 and 1.3. 
         [0029]    In a preferred embodiment, the irradiation for reprogramming is carried out at an output of between 7 and 20 mW with pulse lengths between 5 fs and 20 fs and for a duration of between 0.2 and 1 second. 
         [0030]    It is particularly advisable to carry out the irradiation for reprogramming at an output of between 50 and 100 mW with pulse lengths between 100 fs and 200 fs and for a duration of between 0.2 and 1 second. 
         [0031]    Cells to be reprogrammed are preferably provided as monolayer cells on a glass substrate on the x-y table and are covered with the virus-free cocktail in aqueous solution before irradiating for reprogramming 
         [0032]    In an alternative variant of the method, cells to be reprogrammed are streamed as aqueous cell suspension with the virus-free cocktail into a microfluidic flow cell through a micro-cannula. The irradiation for reprogramming is preferably carried out with an output between 50 mW and 100 mW using a shaped laser beam in Bessel beam mode with elongated focus, the laser beam forms an elongated focus over the entire diameter of the micro-cannula, and the scanner of the laser scanning microscope carries out a line scan orthogonal thereto so as to cover an entire cross-sectional area of the micro-cannula, and the cell suspension flows through the micro-cannula with a flow rate of 135 to 145 nl/s. 
         [0033]    In this connection, it is advisable that a flow is circulated through the flow cell so that the cell suspension of cells to be reprogrammed and virus-free cocktail can stream through the micro-cannula repeatedly (up to three repetitions) in order to increase the hit ratio of cells to be reprogrammed with the scanned elongated focus. 
         [0034]    After a diffusion time of at least five seconds after irradiation, the plasmid cocktail around the cells to be reprogrammed is advantageously replaced by plasmid-free medium, and the cells are incubated and stored in the plasmid-free medium in an incubator for at least two days. 
         [0035]    It has proven advisable to monitor the results of the optical reprogramming during storage in the incubator by means of a fluorescence microscope through evidence of a green fluorescent protein which has been co-transfected during reprogramming through the transient hole in the cell membrane. 
         [0036]    An optical multiple reprogramming of the at least one cell to an iPS cell is advantageously carried out by means of optical treatment. 
         [0037]    In a further advantageous embodiment, a direct optical reprogramming can be carried out by means of the optical treatment of the at least one cell through conversion of one cell type into another cell type. 
         [0038]    Further, the optical treatment can be utilized in a tissue formed of a three-dimensional cell complex as perforation by boring channels with a diameter of up to 10 μm. 
         [0039]    The above-stated object is further met in an apparatus for reprogramming living cells in which a cocktail made up of a required microRNA and at least two plasmids as transcription factors for the reprogramming is transfected into the interior of at least one cell to be reprogrammed in order to convert it into iPS cells or into another type of cell, which apparatus is characterized in that a femtosecond laser is provided for irradiation at a frequency between 75 and 85 MHz and a center wavelength between 750 and 900 nm to generate a virus-free optical reprogramming through selective perforation of a cell membrane for transfecting the cocktail into the interior of the at least one cell to be reprogrammed, and in that there is provided a laser scanning microscope which is outfitted with the femtosecond laser and which has a microscope objective with a high numerical aperture between 0.9 and 1.5, with respect to which laser scanning microscope the cells to be reprogrammed can be arranged such that cells to be reprogrammed can be continuously selected for irradiation in order to achieve a perforation with at least one transient small-pore hole having a size in the range of up to 500 nm within the cell membrane of the at least one cell to be reprogrammed. 
         [0040]    An x-y table is preferably provided by which the positioning of monolayer cells on a glass substrate can be carried out for focusing a laser beam on the cell membrane of the monolayer cells. 
         [0041]    The femtosecond laser is advisably configured as a femtosecond laser with a frequency between 75 and 85 MHz, a pulse length between 10 fs and 20 fs and a center wavelength between 750 and 900 nm and which can be focused on the cell membrane by means of the microscope objective having a high numerical aperture in the range of from 1.1 to 1.3 with a focus in the submicrometer range of up to 500 nm. 
         [0042]    In a particularly advantageous manner, the x-y table has a microfluidic flow cell with a micro-cannula through which flows an aqueous cell suspension of the cocktail with the cells to be reprogrammed. 
         [0043]    The femtosecond laser is preferably configured to emit a Bessel beam with an elongated focus, and the diameter of the micro-cannula is fully covered by an elongated focus of the Bessel beam that is moved in a line scan by means of a scanner of the laser scanning microscope such that the cells to be reprogrammed pass through the focus at a flow velocity generated by a flow rate between 135 and 145 nl/s with a cannula diameter of 100 μm and are impinged upon while flowing through. 
         [0044]    A cell chamber is advisably arranged downstream of the flow cell to capture the cell suspension of reprogrammed cells and cocktail and for replacing the cocktail with a plasmid-free medium for storage in an incubator. 
         [0045]    The invention is based on the fundamental idea that the efficiency of cell reprogramming can be decisively increased through the use of optical reprogramming based on ultrashort laser pulses for transfecting the required DNA plasmids and microRNA. In this respect, a transient alteration of the permeability of the cell membrane is brought about in that the latter is perforated by fs laser pulses and transcription factors for reprogramming are transferred into the interior of the cell. 
         [0046]    Ultrashort laser pulses are used on principle for the optical cell reprogramming according to the invention as is known for laser-based permanent transfection of DNA into living cells (U.S. Pat. No. 7,892,837 B2), wherein the laser causes transient membrane holes to be generated. A flow cytometer for femtosecond laser perforation of cells has also been described for this purpose in WO 2013/120960 A1. Heretofore, however, only one gene has been transfected by laser action, generally a plasmid which produces a green fluorescence (GFP protein). However, reprogramming requires transfection of multiple genes or reprogramming factors (usually four for producing iPS cells). On the other hand, the use of lasers instead of problematic viral reprogramming of cells is a completely novel approach which, as “sterile optical reprogramming”, permits low-risk reprogramming of cells also in three-dimensional cell complexes and potentially within the human body. 
         [0047]    Preliminary work has shown that an optical, direct, virus-free reprogramming, particularly for producing human iPS cells from human fibroblasts, is possible using femtosecond lasers. For this purpose, the human cells were added to a medium containing four transcription factors, Oct-4, NANOG, Lin-28 Sox2 and a GFP plasmid (green fluorescent protein as marker), and were irradiated with twelve femtosecond pulses of a 85-MHz titanium : sapphire laser for a few milliseconds (50-100 ms). Astonishingly, a single bombardment of the cells in a special flow cytometer is sometimes sufficient to generate a plurality of iPS colonies which exhibit green fluorescence as a result of the additional transfection of the GFP plasmid. Interestingly, iPS cell clusters (embryoic bodies) were already generated after 3 to 5 days and, therefore, considerably faster compared to viral direct reprogramming Aside from the achieved rapidity, the high efficiency of the optical reprogramming compared to viral methods is surprising. Accordingly, iPS cells can be produced on the one hand or a direct conversion of one cell type into another cell type (i.e., so-called direct reprogramming) can take place, which has only been partially successful in the art to date (see, e.g., Efe et al., Vierbuchen et al., both cited above, Szabo et al.: Direct conversion of human fibroblasts to multilineage blood progenitors,  Nature  485 [2012] 585, or Kim: Converting human skin cells to neurons: a new tool to study and treat brain disorders?,  Cell Stem Cell  9 [2011] 179). 
         [0048]    In principle, the laser system for optical reprogramming can also be employed directly in three-dimensional collections of cells. To this end, transcription factors must be introduced into the microenvironment. This can be carried out, e.g., through mechanical injection or, in accordance with the invention, through optical generation of defined microchannels from the surface of the tissue to the target site in the tissue by means of a femtosecond laser system. The femtosecond laser system shall then also be utilized to produce the required membrane pores for transfecting the transcription factors from the microenvironment into the cell. Accordingly, target tissue can be reprogrammed in a spatially selective manner This is usually carried out outside the human body, e.g., within the framework of tissue engineering. In principle, however, the method according to the invention can also be utilized within the body of the patient. 
         [0049]    Heretofore, medically compatible femtosecond laser systems such as multiphoton tomographic apparatus for skin analysis and systems for the treatment of vision defects have not been suitable for enabling optical reprogramming in patient tissue. In tomography equipment, typical 80-MHz laser pulses with a length of 100 fs to 200 fs and an average output of typically 15 mW with conventional radiation dwell times of less than 100 μs are only suitable for generating exclusively visual imaging of the tissue through two-photon fluorescence or frequency doubling (second harmonic generation—SHG). By contrast, femtosecond lasers for the treatment of defective vision rely on destructive, photodisruptive effects by means of high-energy femtosecond laser pulses in the kHz range by which plasma-filled bubbles and shockwaves are generated which destroy tissue structures, including the stable collagen network and whole cells. Selective action upon individual cells in order to bore a transient channel into the membrane of the cell without causing irreversible damage to the cell is definitely impossible. 
         [0050]    The optical, virus-free, complete or partial reprogramming of cells in accordance with the invention opens up entirely novel therapeutic possibilities, particularly in the field of regenerative medicine (e.g., the production of graft tissues) and in the treatment of cancers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    The invention will be described more fully in the following with reference to embodiment examples and drawings. The drawings show: 
           [0052]      FIG. 1  is a schematic view of an arrangement according to the invention for implementing the method; 
           [0053]      FIG. 2  is a schematic view of a first embodiment form of an apparatus according to the invention for treating a monolayer cell complex; 
           [0054]      FIG. 3  is a schematic view of a second embodiment form of an apparatus according to the invention with a flow cell device, including a possible multiple treatment of the cells flowing through; 
           [0055]      FIG. 4  is an enlarged detail of the flow cell according to  FIG. 3  showing a scanned laser beam in the form of a Bessel beam with elongated focus. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]      FIG. 1  shows the basic construction of an apparatus according to the invention comprising a radiation source  1 , a laser scanning microscope  2  and a control unit  3 . The laser scanning microscope  2  has a x-y table  27  on which a sample  5  is held and which can be moved in a horizontal x and y direction. In order to observe the sample  5  through a microscope objective  25 , the laser scanning microscope  2  has illumination  28  and a video camera  29 . The movement of the x-y table  27  is carried out by means of an x-y table drive  26 . The movement of the microscope objective  25  for focusing in a vertical z direction is carried out by means of a focusing drive  24 . The x-y table drive  26  and focusing drive  24  are connected to the control unit  3 . 
         [0057]    The radiation source  1  delivers a pulsed laser beam  16  with pulse lengths in the focus of the laser scanning microscope  2  in the range of 5 to 250 fs, preferably in the range of between 5 and 200 fs, particularly preferably between 5 and 30 fs, and a high pulse repetition frequency in the range of from 50 MHz to 2 GHz, preferably in the range of between 70 and 1000 MHz. In a particularly preferred variant, the laser beam  16  has pulse lengths of 10 to 20 fs and a pulse repetition frequency of between 75 and 85 MHz. 
         [0058]    Cell membranes of cells in the sample  5  are transiently perforated by the laser beam  16 . To this end, the laser beam  16  is deflected along the beam path by means of a scanner  21  and, after passing through a beam expansion  22 , is deflected by a beamsplitter  23  in the direction of the sample  5 , where it is focused in the sample  5  through the microscope objective  25  with a very high numerical aperture between 0.9 and 1.5, preferably between 1.1 and 1.3. The scanner  21  moves the focused laser beam  16  over a determined region of the sample  5  such that the quantity of cells impinged by the focused laser beam  16  can be increased. The beamsplitter  23  allows the sample  5  to be observed while the laser beam  16  is used. The timing and power of the laser beam  16  are likewise controlled by the control unit  3 . 
         [0059]    The laser beam  16  is controlled by a shutter  12  for temporally limiting the laser action on the cell membrane and for boring channels in a cell complex (hereinafter, tissue) and by a control unit  3  for power switching between a nondestructive radiation mode of the laser beam  16  for adjustment and observation of the cells and a perforation mode of the laser beam  16  for generating transient small-pore holes in the cell membrane (pore size 10 nm to 500 nm) for purposes of transferring (temporary diffusion) microRNA and transcription factors (in the form of plasmids, episomal vectors, transposons). 
         [0060]    In order to achieve small-pore holes of this kind in the cell membrane, the laser beam  16  is influenced with respect to the light distribution thereof before entering the laser scanning microscope  2  by a beam-shaping unit which can include a dispersion compensator  13 , preferably in the form of chirped mirrors, an Axikon element  14  for generating a coaxial illumination ring, and periscope optics  15  as shown by way of example in  FIG. 2 . Further, the laser beam  16  is influenced by an attenuation unit  17  for controlling the output power of the laser. All of these units improve the homogeneity and radial intensity distribution toward an increase in the edge intensity of the initial Gaussian bundle for sharper focusing of the laser beam  16  in the target volume of the sample  5  and its output power for the observation mode and for the perforation mode. 
       EMBODIMENT EXAMPLE 1 
       [0061]    In a first embodiment example, human skin cells marketed by LONZA (#CC-2511) are cultured as monolayer cells in a supply receptacle  4 . For the reprogramming of cells, a solution covering the monolayer cells  51  is added to the supply receptacle  4 . The solution contains a plasmid mixture marketed by SBI (System Biosciences pMC-LGNSO MiniCircle DNA, #SRM100A-1) with plasmids Oct-4, Lin-28, NANOG, Sox2+GFP in a concentration of 5-10 μg/ml. 
         [0062]    Human skin cells marketed by LONZA (#CC-2511) were cultured as monolayer cells  51  in a supply receptacle  4  with a glass bottom having a thickness of 160 μm which produces a working distance of 170 μm from the cells to be treated. A suspension in the form of a plasmid cocktail marketed by SBI (System Biosciences, pMC-LGNSO MiniCircle DNA, #SRM100A-1) containing plasmids Oct-4, Lin-28, NANOG, Sox2+GFP in a concentration of 5-10 μg/ml is added as solution to the monolayer cells  51  for biochemical implementation of the reprogramming step. The thickness of the glass bottom of the supply receptacle  4  corresponds to approximately 40 times the numerical aperture of the microscope objective  25  which, in this case, has a numerical aperture of 1.3. 
         [0063]    The cells which have been prepared in this way are then exposed by means of a laser scanning microscope  2  to irradiation by a femtosecond laser  11  (10 fs, 85 MHz, center wavelength 800 nm) by means of a microscope objective  25  with a high numerical aperture of 1.3. 
         [0064]    In principle, the femtosecond laser  11  can preferably have a pulse repetition frequency of between 80 and 85 MHz and pulse lengths of between 10 and 250 fs and can be used with wavelengths ranging from 700 to 1200 nm When a pulse length of 100 to 200 fs is used in the focus of the laser beam  16 , a mean power of 50 to 100 mW must be set; however, with pulse lengths between 10 and 20 fs a mean power of only 7 to 15 mW is adjusted so that the individual cells are not destroyed during the perforation of the cell membrane. 
         [0065]    To find a suitable membrane position in a selected cell, a motor-driven x-y table  27  is moved in such a way that the focus of the attenuated, nondestructive laser beam  16  is positioned on the cell membrane. The attenuated, nondestructive laser beam  16 ′ which is required solely for imaging by means of video camera  29  is operated at an output below 5 mW. 
         [0066]    The mean output of the laser beam  16  is then increased to approximately 10-15 mW and the membrane is irradiated for 50-100 ms. In this way, it is also possible to perforate an individual cell at up to three positions for each individual exposure. 
         [0067]    The destructive effect of boring a transient hole with a diameter in the range of from 10 to 500 nm is achieved through the formation of a plasma-filled cavitation bubble. It is brought about by means of a flash vaporization of the volume of the cell membrane located in the focus and is recorded by the video camera  29 . The cavitation bubble which has a maximum size of 5 μm disappeared in some experiments after approximately 5 seconds. Within this time, the plasmid cocktail was able to diffuse into the cell. 
         [0068]    After irradiation, the cocktail medium is exchanged for a plasmid-free medium and the cells are stored in the incubator  7  under a gas atmosphere of 5% CO 2  and 95% air at 37° C. Proof that the plasmids have been taken into the cell DNA can be furnished by the formation of the added green fluorescent GFP protein using a fluorescence microscope. The green fluorescence usually occurs within 12 to 36 seconds after laser irradiation. Three-dimensional green fluorescent cell clusters arise over the course of the next five days with a morphology corresponding to that of virus-generated cell clusters (embryoic bodies). 
       EMBODIMENT EXAMPLE 2 
       [0069]    Human skin cells marketed by LONZA (#CC-2511) in a cell suspension  52  containing a plasmid cocktail marketed by SBI (System Biosciences), pMC-LGNSO MiniCircle DNA, #SRM100A-1 with plasmids Oct-4, Lin-28, NANOG, Sox2+GFP (in a concentration that is three to four times higher than that applied to the monolayer cells  51  in Example 1) are added to a receptacle of a metering device  6 . 
         [0070]    In an apparatus shown in  FIG. 2 , the cell suspension  52  is supplied to a flow cell  42  through line  41  from the metering device  6  which can be a conventional syringe with a linear plunger feed. The flow cell  42  comprises a micro-cannula  45  which in this example has an inner diameter of 100 μm and in which the skin cells are virtually isolated, if permitted by the cell suspension  52 , to flow through the micro-cannula  45  at a typical flow velocity of 18 μm/ms and a flow rate of 139 nl/s. 
         [0071]    After passing through the microscope optics  25  of a laser scanning microscope  2 , a laser beam  16  of the femtosecond laser  11  having a beam profile shaped into a quasi-Bessel beam by the beam-shaping unit comprising dispersion compensator  13 , Axikon element  14  and periscope optics  15  has an elongated focus over the entire diameter of the micro-cannula  45  and impinges with a repetition frequency of 80 MHz such that it is constantly perpendicular to the direction of the micro-cannula  45  and, in so doing, is moved by the scanner  21  orthogonal thereto in a line scan at 7 to 30 ms per line so as to permeate an inner cross-sectional area of the micro-cannula  45  in a continuous, practically two-dimensional manner and accordingly perforates a majority of the cells to be reprogrammed (e.g., human skin cells) inside micro-cannula  45  of flow cell  42 . The maximum mean output of the laser beam  16  is 135 mW in quasi-Bessel beam mode (utilizing a 10×, 1.13 NA objective for focusing the laser pulses over the entire inner cross-sectional area of the micro-cannula  45 ). 
         [0072]    To illustrate the continuous permeation of the micro-cannula  45  by the scanned elongated laser focus as the cell suspension  52  is streamed through,  FIG. 4  shows a schematic view of a section of the micro-cannula  45  in plane A-A. The microscope objective  25  (not shown) is directed from above onto plane A-A in the drawing. The elongated focus extends perpendicular to the drawing plane and covers the inner diameter of the micro-cannula  45  in depth direction (downward in z direction). The y direction is swept by the scanner  21  of the laser scanning microscope  2  vertical to the drawing plane and leads to a permanent quasi-two-dimensional formation of the focus over the entire inner cross-sectional area of the micro-cannula  45 . In this way, with streaming cell suspension  52 , a high efficiency of the optical reprogramming is already achieved through progressive perforation of the cells in the stream of cell suspension  52 . 
         [0073]    All the rest of the parameters and processes of the laser irradiation are carried out in the same manner as in Example 1. 
         [0074]    The cells are subsequently removed from the micro-cannula  45  to a standardized cell chamber  44  via an outlet  43  and are captured therein and then washed in a growth medium and incubated and stored with the growth medium in the incubator  7  at 37° C. under 5% CO 2  and 96% air. The cells which exhibit green fluorescence as a result of successful transfection are then detected by a fluorescence microscope and separated by centrifuging after the usual period of two to five days. 
         [0075]    In a modified apparatus according to  FIG. 3  in which only the construction of the flow cell  42  has been modified compared to  FIG. 2 , the cell suspension  52  flows through the micro-cannula  45  multiple times. To this end, a circulation system  46  of tubes is connected to the micro-cannula  45 , a pump being installed therein so that the cell suspension  52  which has already been irradiated once flows through the micro-cannula  45  with two to three repetitions. A higher efficiency of the yield of cells which are optically reprogrammed multiple times can be achieved in this way. Since the cell membrane of a cell can also be perforated repeatedly without fatal damage to the cell, repeated consecutive perforation also does not pose an additional risk for the cells to be reprogrammed All of the rest of the steps and processes are carried out in the same manner as described with reference to  FIG. 2 . 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  radiation source 
           11  femtosecond laser 
           12  shutter 
           13  dispersion compensator (chirped mirror) 
           14  Axikon element 
           15  periscope optics 
           16  laser beam 
           16 ′ laser beam 
           17  attenuation unit 
           2  laser scanning microscope 
           21  scanner 
           22  beam expansion 
           23  beamsplitter 
           24  focusing drive (z direction) 
           25  microscope objective 
           26  x-y table drive (x-y direction) 
           27  x-y table 
           28  illumination 
           29  video camera 
           3  control unit 
           4  supply receptacle 
           41  inlet line 
           42  flow cell 
           43  outlet line 
           44  cell chamber 
           45  micro-cannula 
           46  circulation system 
           5  sample 
           51  monolayer cells 
           52  cell suspension 
           6  metering device 
           7  incubator