POLYPEPTIDE DELIVERY II-2

Abstract: We describe a method of preparing a red blood cell vehicle suitable for delivering an agent to a target site in a vertebrate, the method comprising the steps of: (a) providing a red blood cell; and (b) loading the red blood cell with an agent-MTS conjugate.

Description: POLYPEPTIDE DELIVERY II-2 FIELD OF THE INVENTION The present invention relates to the delivery of an agent to a target site. In particular, the present invention relates to methods for delivering an agent in a red blood cell loaded with the agent, which cell may be sensitised to assist in agent release. BACKGROUND TO THE INVENTION The delivery of therapeutic agents to specific tissues is desirable typically to ensure that a sufficiently high dose of a given agent is delivered to a selected tissue. Examples of therapeutic agents which are currently sought to be delivered include antibodies, enzymes, transcription factors, nucleic acids (DNA, RNA, etc), ribozymes, oligonucleotides, peptides and aptamers, among others. The target location where it is intended for the agent to act is, however, typically within the cell (for example, within the nucleus if the agent is to affect gene transcription). However, therapeutic agents of interest, for example, those listed above, typically cross cell membranes at low efficiency. A particular problem therefore arises in ensuring the agent is delivered into the intracellular environment of a cell. The failure of agents to penetrate cell membranes may be due to various factors, such as their intrinsic size, charge, polarity and chemical composition. A number of different methods have been developed for the delivery of agents into cells. For example, direct micro-injection of the agent into cells of interest may be used. Furthermore, modified viruses have also been proposed as delivery vehicles or vectors. For example, viruses ' such as adeno associated virus (AAV), adenovirus, baculovirus, retroviruses, modified Semliki Forest Virus (SFV), lentiviruses (such as HIV) and herpesvirus (such as Herpes Simplex Virus, HSV) have been proposed as vectors for intracellular delivery of agents. Thus, viral agents have been used to deliver agents in gene therapy. Furthermore, it has been suggested that agents may be delivered into cells as protein fusions or conjugates with a protein capable of crossing the plasma membrane and/or the nuclear membrane. Known domains and sequences having translocational activity include sequences from the HIV- 1 -trans-activating protein (Tat), Drosophila Antennapedia homeodomain protein and the herpes simplex- 1 virus VP22 protein. Although methods of delivery of agents into cells have been proposed, no one to our knowledge has disclosed particular methods of targeting the delivery vehicles to appropriate tissues or organs. Thus, the delivery methods proposed previously rely on general systemic administration of the delivery vector, or on injection to a particular part of a patient's body. Each of these techniques has its disadvantages, including wastage, imprecise targeting, and takeup at inappropriate sites, leading to unwanted side effects. In the case of injection, there is restriction of targeting to accessible sites of the patient, and surgical intervention may be needed to target internal sites. SUMMARY OF THE INVENTION The present invention seeks to overcome the problems associated with the prior art methods of delivery. The invention is based on the discovery that it is possible to utilise membrane translocation sequences, conjugated to agents of interest, to load red blood cell delivery vehicles. We find that such loading is effected by exposing red blood cells to the agent-MTS conjugate, such that the agent-MTS conjugate automatically loads itself into the vehicle. We find that the loaded agent-MTS conjugates do not substantially leak from the red blood cell; i.e., that such loaded red blood cells substantially retain the agent-MTS conjugates effectively. We find that the loaded red blood cells may be subjected to an optional sensitisation step, preferably, an electrosensitisation step, to render them more susceptible to disruption by exposure to an energy source. Such sensitised loaded red blood cells (and red blood cells sensitised before loading by agent-MTS conjugates) may be subsequently disrupted by a administration of energy, for example, ultrasound. The agent-MTS conjugates are released in an active state, and are taken up by adjacent cells. Accordingly, our invention enables the local release and delivery (including takeup) of agents to be accomplished. According to a first aspect of the present invention, we provide a method of preparing a red blood cell vehicle suitable for delivering an agent to a target site in a vertebrate, the method comprising the steps of: (a) providing a red blood cell; and (b) loading the red blood cell with an agent-MTS conjugate. Preferably, the method further comprises the step of sensitising the red blood cell, whether before or after the loading step, to render the red blood cell more susceptible to disruption by exposure to a stimulus than an unsensitised red blood cell. There is provided, according to a second aspect of the present invention, a method of preparing a red blood cell vehicle suitable for delivering an agent to a target site in a vertebrate, the method comprising the steps of: (a) providing a red blood cell loaded with an agent-MTS conjugate; and (b) sensitising the red blood cell. We provide, according to a third aspect of the present invention, a method of preparing a red blood cell vehicle suitable for delivering an agent to a target site in a vertebrate, the method comprising the steps of: (a) providing a sensitised red blood cell; and (b) loading the red blood cell with an agent-MTS conjugate. As a fourth aspect of the present invention, there is provided a method for delivering an agent to a target site in a vertebrate, comprising the steps of: (a) providing a red blood cell; (b) loading the red blood cell with an agent-MTS conjugate; (c) sensitising the red blood cell to render it more susceptible to disruption than an unsensitised red blood cell; (d) introducing the red blood cell into a vertebrate; and (e) causing the agent-MTS conjugate to be released from the sensitised red blood cell by applying energy to the sensitised red blood cell; in which steps (b) and (c) may be performed in any order. We provide, according to a fifth aspect of the present invention, a red blood cell vehicle suitable for delivery of an agent to a vertebrate, the red blood cell comprising an agent-MTS conjugate. Preferably, the red blood cell is sensitised so that it is rendered more susceptible to disruption by exposure to a stimulus than an unsensitised red blood cell. In any of the above aspects of the invention, the red blood cell may be sensitised by applying an electric field to the red blood cell. Preferably, the electric field has a field strength of from about O.lkVolts/cm to about 10 kVolts/cm under in vitro conditions. More preferably, the red blood cell is sensitised by application of an electric pulse for between lμs and 100 milliseconds. Most preferably, the red blood cell is sensitised in such a way as to be capable of being disrupted by exposure to ultrasound. The ultrasound may be selected from the group consisting of diagnostic ultrasound, therapeutic ultrasound and a combination of diagnostic and therapeutic ultrasound. The applied ultrasound energy source is preferably at a power level of from about 0.05 /cm 2 to about lOOW/cm 2 . Furthermore, the red blood cell vehicle may be pre-sensitised so that it is capable of being loaded with a larger amount of agent than a red blood cell which has not been pre-sensitised. Preferably, the pre-sensitisation comprises exposing the red blood cell to an electric field and/or ultrasound. The membrane translocation sequence may be any sequence which enables the agent to cross the plasma membrane of a cell. Preferably, the agent comprises a fusion protein, in which the polypeptide is fused to a membrane translocation sequence. In a preferred embodiment of the invention, the membrane translocation sequence comprises a sequence selected from: HIV- 1 -trans-activating protein (Tat), Drosophila Antennapedia homeodomain protein (Antp-HD), Herpes Simplex- 1 virus VP22 protein (HSV-VP22), signal-sequence-based peptides, Transportan and Amphiphilic model peptide. The membrane translocation sequence may further comprise homologues of the any of the foregoing, and fragments, variants and mutants having membrane translocational activity. In a highly preferred embodiment of the invention, the membrane translocation sequence comprises the sequence GRKKRRQRRRPPQC, RQIKIWFQNRRMKWKK or RQIKIWFQNRRMKWKKC. The agent to be delivered may be selected from a group consisting of a biologically active molecule, a protein, a polypeptide, a peptide, a nucleic acid, an oligonucleotide, a peptide nucleic acid (PNA), a virus-like particle, a nucleotide, a ribonucleotide, a deoxyribonucleotide, a modified deoxyribonucleotide, a heteroduplex, a nanoparticle, a synthetic analogue of a nucleotide, a synthetic analogue of a ribonucleotide, a modified nucleotide, a modified ribonucleotide, an amino acid, an amino acid analogue, a modified amino acid, a modified amino acid analogue, a steroid, a proteoglycan, a lipid, a carbohydrate, and mixtures, fusions, combinations or conjugates of the above. The agent to be delivered may be conjugated to, fused to, mixed with or combined with an imaging agent. The present invention, in a sixth aspect, provides the use of a red blood cell prepared according to any of the first to fourth aspects of the invention, or a red blood cell according to the fifth aspect of the invention, in the preparation of a medicament for delivery of an agent to or at a target site. According to a seventh aspect of the present invention, we provide the use of a red blood cell prepared according to any of the first to fourth aspects of the invention, or a red blood cell according to the fifth aspect of the invention, for the delivery of one or more agents to a vertebrate. Such delivery may be accomplished by active release from the red blood cell vehicle by application of a stimulus to disrupt the red blood cell vehicle. We provide, according to an eighth aspect of the invention, a kit comprising a red blood cell prepared according to any of the first to fourth aspects of the invention, or a red blood cell according to the fifth aspect of the invention, an agent-MTS conjugate suitable for loading into said red blood cell and packaging materials therefor. There is provided, in accordance with a ninth aspect of the present invention, a pharmaceutical composition comprising a red blood cell prepared according to any of the first to fourth aspects of the invention, or a red blood cell according to the fifth aspect of the invention, together with a physiologicaly compatible buffer. As an tenth aspect of the invention, we provide a method of loading a red blood cell with an agent, the method comprising the steps of: (a) providing a red blood cell; and (b) exposing the red blood cell to an agent-MTS conjugate. We provide, according to a eleventh aspect of the invention, there is provided use of a membrane translocation sequence in a method of delivery of an agent to a vertebrate, in which the method comprises the steps of: (a) providing an agent -to be delivered; (b) joining the agent to a membrane translocation sequence to produce an agent-MTS conjugate; and (c) loading the agent-MTS conjugate into a red blood cell vehicle. Further aspects of the invention include the following: a method for delivering an agent to a target site in a vertebrate, comprising the steps of: (a) providing a red blood cell; (b) loading the red blood cell with an agent-MTS conjugate; (c) introducing the red blood cell into a vertebrate; and (d) causing the agent-MTS conjugate to be released from the sensitised red blood cell. According to a yet further aspect of the present invention, we provide a method for the immunisation of an animal with an antigen, the method comprising the steps of: (a) providing a red blood cell; (b) loading the red blood cell with an antigen; (c) introducing the red blood cell into a vertebrate; and (d) causing the agent to be released from the red blood cell. Preferably, the red blood cell is sensitised, more preferably, electrosensitised, to render the red blood cell more susceptible to disruption by exposure to a stimulus than an unsensitised red blood cell. Preferably, the red blood cell is disrupted by exposure to ultrasound. Preferably, steps (c) and/or (d) are repeated. The antigen may be provided in the form of an agent-MTS conjugate comprising the antigen agent. There is provided according to a further aspect of the invention, the use of a membrane translocation sequence to load an agent into a red blood cell. A further aspect of the invention provides for a method of producing a red blood cell suitable for delivery of a polypeptide to a vertebrate, the method comprising the steps of: (a) providing a transgenic animal carrying and expressing a transgene encoding a fusion protein comprising the polypeptide and a membrane translocation sequence (MTS); (b) obtaining a red blood cell containing the fusion protein from the animal; and (c) sensitising the red blood cell sensitising the red blood cell to render it susceptible to disruption by an energy source. A yet further aspect of the invention provides for a method of producing a red blood cell suitable for delivery of a polypeptide to a vertebrate, the method comprising the steps of: (a) providing a red blood cell containing a polypeptide, the red blood cell being derived from a transgenic animal carrying and expressing a transgene encoding a fusion protein comprising the polypeptide and a membrane translocation sequence (MTS); and (b) sensitising the red blood cell to render it susceptible to disruption by an energy source. Another aspect of the invention provides for a method for the delivery of a polypeptide to a vertebrate, the method comprising the steps of: (a) providing a transgenic animal carrying and expressing a transgene encoding a fusion protein comprising the polypeptide and a membrane translocation sequence (MTS); (b) obtaining a red blood cell containing the fusion protein from the animal; (c) sensitising the red blood cell to render it susceptible to disruption by an energy source; (d) introducing the sensitised red blood cell to a vertebrate; and (e) exposing the vertebrate, or a part of it, to an energy source at a level sufficient to disrupt the sensitised red blood cell. In another aspect of the invention , there is provided a method of producing a polypeptide agent-MTS conjugate, the method comprising the steps of: (a) isolating a red blood cell from a transgenic animal carrying and expressing a transgene encoding a fusion protein comprising the polypeptide and a membrane translocation sequence (MTS); (b) sensitising the red blood cell to render it susceptible to disruption by an energy source; (c) exposing the red blood cell to an energy source sufficient to disrupt the sensitized red blood cell; and (d) isolating the fusion protein to provide the polypeptide agent-MTS conjugate. The transgenic animal is preferably selected from the group consisting of: mouse, rat, rabbit, sheep, goat, cow, and pig. More preferably, the polypeptide is expressed under the control of a β-globin promoter or enhancer. Most preferably, the polypeptide is expressed under the control of a β-globin Locus Control Region (LCR). BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a diagram showing the autoloading of electrosensitised rabbit cells with FITC-labelled HIV-TAT fragment. 1= control; 2 = 0 mg/ml; 3 = 0.05 mg/ml; 4 = 0.1 mg/ml; 5 = 0.2 mg/ml; 6 = 0.3 mg/ml; 7 = 0.4 mg/ml; 8 = 0.5 mg/ml. Figure IB is a diagram showing the autoloading of electrosensitised rabbit cells with FITC-labelled penetratin. 1 = 0 mg/ml; 2 = 0.01 mg/ml; 3 = 0.03 mg/ml; 4 = 0.06 mg/ml; 5 = 0.1 mg/ml. Figure 1C is a diagram showing the autoloading of electrosensitised rabbit cells with FITC-labelled VP-22. 1= control; 2 = 0 mg/ml; 3 = 0.1 mg/ml; 4 = 0.2 mg/ml; 5 = 0.3 mg/ml; 6 = 0.4 mg/ml; 7 = 0.5 mg/ml. Figure 2 A is a diagram showing the stability of HIV-TAT fragment autoloaded human cells in whole blood . Figure 2B is a diagram showing the stability of HIV-TAT fragment autoloaded rabbit cells in whole blood. Figure 2C is a diagram showing the stability of HIV-TAT fragment autoloaded pig cells in whole blood . Figure 2D is a diagram showing the stability of HIV-TAT fragment autoloaded mouse cells in whole blood . Figure 3 is a diagram showing a flow cytometric analysis of a lymphocyte population: Red - lymphocytes alone, Green - lymphocytes with Penetratin 0.1 mg/ml; Blue - lymphocytes with ultrasound lysate from RBC loaded Penetratin, concentration 0.1 mg/ml. Figure 4 is a flow cytometric analysis showing the loading of FITC-labelled penetratin-oligonucleotide conjugate into sensitised human red blood cells. Figure 5. Flow cytometric analysis of dialysis loading of HIV-TAT fragment in pig erythrocytes. X-axis: FLH-1, Y axis: counts. Figures 6A and 6B. Stability of loaded pig red blood cells in whole blood. X- axis: time in hours; Y axes: percentage cells and geometric mean. 6A 4 degrees C, 0.05 mg/ml 2 nd population; 6B 37 degrees C, 0.05 mg/ml 2 nd population (loaded vehicle). Figure 7A. Events in the M2 region (loaded vehicle) from electrosensitised, dialysed, HIV-TAT fragment loaded pig cells subjected to varying ultrasound intensities in the circulating phantom. X-axis: time in minutes; Y-axis: events in the M2 region. Figure 7B. Haemoglobin release from electrosensitised, dialysed HIV-TAT fragment loaded pig cells subjected to varying ultrasound intensities in the circulating phantom. X-axis: time in minutes; Y-axis: OD at 540nm. Figure 7C. Haemoglobin release from non-electrosensitised, dialysed HIV- TAT fragment loaded pig cells subjected to varying ultrasound intensities in the circulating phantom. X-axis: time in minutes; Y-axis: OD at 540nm. Figure 8 A. Graph showing ultrasound mediated release of peptide payload in vivo. Arrows above denote 10 minute applications of ultrasound pulsed wave (35%) at 6W/cm 2 . ' Figure 8B. Effect of ultrasound in circulating phantom upon electrosensitised loaded cells recovered from pig 10 minutes post administration. X-axis: time in circulating phantom at 6W/cm~; Y-axis: cells in Ml region (loaded vehicle). Figure 9. Graph showing the in vivo effect of ultrasound on TAT-FITC loaded pig red blood cells, not electrosensitised. X-axis: time in minutes (ultrasound applications of 3 x 10 minute bursts at 6W/cm are indicated by downward arrows). Y- axis: number of cells in the fluorescent region. Figure 10. Graph showing the in vivo effect of ultrasound on TAT-FITC loaded pig electrosensitised red blood cells. X-axis: time in minutes (ultrasound applications of 8 x 1 minute bursts at 6W/cm 2 pulsed wave are indicated by downward arrows). Y- axis: number of fluorescent cells in the M4 region (i.e., loaded vehicle). Figure 10B. Graph showing the in vivo effect of ultrasound on TAT-FITC loaded electrosensitised pig red blood cells (enlargement of circled section in Figure 10A). X-axis: time in minutes (ultrasound applications of 4 x 1 minute bursts at 6W/cm 2 pulsed wave are indicated by downward arrows). Y-axis: number of fluorescent cells in the M4 region (i.e., loaded vehicle). Figure 11. Graph showing ultrasound mediated release of peptide payload in vivo in pig. X-axis: time in minutes. Y-axis geometric mean of the M2 region(i.e., loaded vehicle). Arrows indicate points when cells are administered and ultrasound applied to the hepatic artery region. Figure 12 A. Graph showing ultrasound mediated changes in M4 cells (loaded vehicle) in vivo in pig. X-axis: time in minutes. Y-axis: events in region. Small arrows denote 30 second applications of ultrasound to the kidney; large arrows denote 1 minute applications of ultrasound to the kidney. Figure 12B. Ultrasound mediated localisation of FITC-labelled TAT in a treated kidney compared to a control untreated organ from the same animal (contralateral kidney). Upper panels: treated renal cortex (1), treated renal medulla (2); lower panels: control renal cortex (1), control renal medulla (2). Figure 13A shows graphs of experiments to establish optimal electrosensitisation cell density conditions for murine erythrocytes. Upper graph: electrosensitisation at 1 x 10 9 cell density. X-axis: voltage in kV. Right hand Y-axis: % lysis with ultrasound. Left hand Y-axis: percentage recovery. Lower graph: electrosensitisation at 1.5 x 10 9 cell density. X-axis: voltage in kV. Right hand Y-axis: % lysis with ultrasound. Left hand Y-axis: percentage recovery. Figure 13B shows graphs of experiments to establish optimal number of pulses during electrosensitisation of murine erythrocytes. Upper graph: electrosensitisation at 1 x 10 9 cell density with one pulse. X-axis: voltage in kV. Right hand Y-axis: % sensitivity. Left hand Y-axis: percentage recovery. Lower graph: electrosensitisation at 1 x 10 9 cell density with two pulses. X-axis: voltage in kV. Right hand Y-axis: % lysis with ultrasound. Left hand Y-axis: percentage recovery. Figure 13C is a flow cytometry profile showing dialysis loading of peptide into murine erythrocytes. Figure 14A shows the effects of ultrasound treatment on loaded mouse cells (M4) in circulating phantom. Mouse cells dialysis loaded with TAT-fragment are subjected to varying ultrasound intensities on the circulating phantom. X-axis: time in minutes. Y-axis: number of cells in M4 region. Filled squares: circulation only; inverted triangles: 4.5 W/cm " ; filled diamonds: 5 W/cm ; circles: 6 W/cm ; upright triangles: 8 W/cm 2 . Figure 14B shows haemoglobin release from electrosensitised, mouse cells dialysis loaded with TAT-fragment and subjected to varying ultrasound intensities in a circulating phantom system. X-axis: time in minutes. Y-axis: OD at 540nm. Filled squares: circulation only; inverted triangles: 4.5 W/cm 2 ; filled diamonds: 5 W/cm 2 ; circles: 6 W/cm 2 ; upright triangles: 8 W/cm 2 . Figure 15A is a graph showing the effect of renal ultrasound treatment on the cell dynamics of loaded cells in a murine model. X-axis: time in minutes; Y-axis: percentage loaded cells. Filled squares: control percentage; upright triangles: ultrasound treated kidney percentages. Figure 15B shows the in vivo effects of ultrasound applied to mouse kidney, following administration of TAT-fragment loaded erythrocytes (approximately 13% spike) into a mouse. Upper panel: treated kidney; lower panel: untreated kidney. Figures 16A and 16B. Binding of oligo, TAT and TAT-oligo conjugate to rabbit aorta, uptake of oligonucleotide, TAT and TAT-oligonucleotide conjugate by rabbit aorta. Samples of each species are placed in contact with the inner surface of rabbit aorta. Tissues are subsequently fixed and paraffin wax sections prepared. Samples are viewed using fluorescence microscopy (A,B & C) for the presence of TAT and with light microscopy (D,E & F) for the presence of biόtinylated oligonucleotide. Figure 16A Panel A: aorta + oligo no DAB, inner surface; Panel B: aorta + FITC-TAT-oligonucleotide-biotin conjugate, inner surface; Panel C: aorta + FITC TAT, inner surface: Panel D: aorta + biotin-oligonucleotide, inner surface. Figure 16B Panel E: aorta + FITC-TAT oligonucleotide-biotin, inner surface; Panel F: aorta+ FITC-TAT, inner surface. Figure 17. Flow cytometry profiles for control unloaded human erythrocytes and erythrocyte preparations loaded with the TAT-oligonucleotide conjugate. Figure 18. Uptake of TAT-oligonucleotide by inner surface of aorta following ultrasound mediated release from loaded human erythrocytes. Fluorescent images obtained from aorta samples placed in contact with PBS (A), TAT-oligonucleotide conjugate-containing lysates (B) and oligonucleotide-containing lysates (C). The latter two lysates are prepared by treating conjugate- and oligonucleotide-containing erythrocytes with ultrasound. Figures 19A and 19B. Uptake of oligonucleotide and TAT-oligonucleotide conjugate by aorta following ultrasound mediated release from human erythrocytes. Light microscopy images obtained from aorta samples placed in contact with lysates containing oligonucleotide (A) and conjugate (B). Lysates are prepared by treating oligonucleotide- and conjugate-containing erythrocytes with ultrasound. Aorta samples are also placed in contact with untreated erythrocytes containing both oligonucleotide (C) and conjugate (D). DETAILED DESCRIPTION OF THE INVENTION According to a general principle of the invention, agents to be delivered to a vertebrate comprise, and/or are coupled, fused, mixed, combined, or otherwise joined to a membrane translocation sequence, and loaded into a red blood cell delivery vehicle for delivery. Membrane translocation sequences are disclosed in further detail below. The coupling, etc between the agent and the membrane translocation sequence may be permanent or transient, and may involve covalent or non-covalent interactions (including ionic interactions, hydrophobic forces, Van der Waals interactions, etc). The exact mode of coupling is not important, so long as the membrane translocation sequence is effective in allowing the agent to cross the cell membrane of a target cell. Accordingly, where reference is made to "comprising", "conjugation", "coupling", etc, these references should be taken to include any form of interaction between the agent to be delivered and the membrane translocation sequence, in such a manner as to allow intracellular delivery of the agent. An agent comprising a membrane translocation sequence, which includes an agent coupled, conjugated, joined, etc to a membrane translocation sequence, is referred for simplicity in this document as an "agent-MTS conjugate". As is clear from the above, " this term includes fusion proteins comprising a membrane translocation sequence and a polypeptide agent to be delivered. Thus, the agent may be a polypeptide which is provided as a fusion protein with the membrane translocation sequence. An expression vector may be constructed by standard recombinant DNA technology to include a nucleotide sequence capable of expressing a membrane translocation sequence, such that a fusion protein is expressed comprising the polypeptide sequence of interest fused to the membrane translocation sequence. The expression vector is transfected or transformed into a suitable host for large scale production of fusion protein, by means known in the art. Purification of the fusion protein may also be carried out by known means. The agent may also be chemically coupled to the membrane translocation sequence. These and other means of joining the membrane translocation sequence to the agent are disclosed in further detail below. The agent-MTS conjugates may be loaded into red blood cells by any suitable means, as described in further detail below. It will be appreciated that, because of the presence of a membrane translocation sequence, the agent-MTS conjugates are capable of crossing the red blood cell membrane and therefore can "self-load" into the red blood cell vehicle with little or no further assistance. Thus, the invention includes a method of loading a red blood cell with an agent, the method comprising exposing a red blood cell to the agent-MTS conjugate. While it is appreciated that energy may be required for membrane translocation of some membrane translocation sequences, for simplicity, such auto-loading of agent-MTS conjugates in RBCs is referred to here as "passive loading" or "autoloading". It will be appreciated however, that "active" loading means may also be employed, in place of, or in conjunction with passive loading. Loading procedures are described in detail below, including a preferred means of active loading using hypotonic dialysis. The RBC vehicles of the invention may be subjected to a pre-sensitising step to increase the efficiency of loading of agent. A preferred pre-sensitising step involves applying an electric field to the red blood cells, as described in our International Patent Application Number PCT/GB00/03056, and also in detail below. The RBC may be further loaded with a second agent, which itself may be an agent-MTS conjugate. Such loading may be active or passive. It will be understood that the invention is not limited to loading of a second agent; third and subsequent agents may also be loaded in the same manner as described here. As described in further detail below, in a highly preferred embodiment of the invention, the loaded red blood cells are sensitised to render them more susceptible to disruption by a stimulus than unsensitised red blood cells. The invention therefore encompasses the use of sensitising agents and/or processes to increase the susceptibility of RBC vehicles according to the invention to disruption using energy such as ultrasound or light energy. The RBC vehicles of the invention are preferably capable of being selectively disrupted at a target site by exposure to a stimulus, for example laser light or ultrasound. Accordingly, "sensitised" is intended to indicate that the cells according to the invention have been treated in order to render them more susceptible to a stimulus. Preferred sensitisation procedures such as electrosensitisation are set forth in our International Patent Application Number PCT/GBOO/02848, and are described in detail below. Such sensitisation may take place during, before or after loading. We have found that agent-MTS conjugates may be loaded effectively into sensitised red blood cells. We have also found that red blood cells loaded with agent-MTS conjugates retain their payloads over the course of several days. The loaded red blood cells (optionally sensitised) are subsequently introduced into a recipient animal, as described in detail elsewhere in this document. Lysis by ultrasound or other energy means enables release of the agent, which is then able to enter the cells in the surrounding tissue. Disruption may be focused in a single tissue, or may be generalised throughout the body. Equally, the energy levels used may be intended to release the contents of substantially all of the RBC vehicles, or only part of these. In the second case, repeated applications of the required energy may be used to provide further doses of the relevant agent. The present invention is useful for the delivery of therapeutic or diagnostic agents to specific sites in vertebrate organisms, without the problems associated with agents being unable to cross the cell membrane. Preferably, the agent is able to cross the plasma or cell membrane of a cell in the vicinity of the area of release. Preferably, the agent is released in the bloodstream (including a capillary), and is internalised within a cell adjacent to the bloodstream. Such a cell may include a cell lining a blood vessel, such as an endothelial cell. The agent may further be capable of being internalised by cells distal to the point of release. Thus, in a particular embodiment, the agent is taken up by endothelial cells, and further is internalised by adjacent cells such as muscle cells. Preferably, the agent is capable of being released and internalised in an in vivo environment, e.g., within the body of an organism such as a human being. The preferred target sites for agent delivery are preferably intracellular sites, for example, specific sub-cellular compartments including organelles such as chloroplasts, mitochondria, nucleus, endoplasmic reticulum, etc. The ability to selectively disrupt RBC vehicles according to the invention permits the person skilled in the art to achieve release of the contents of the RBC at any desired site to which the stimulus required may be directed. The RBC vehicles loaded with agent-MTS conjugate according to the invention may be used for a variety of purposes. Advantageously, the RBC vehicles of the invention are useful for the delivery of agents (as agent-MTS conjugates) to the body of a vertebrate. RED BLOOD CELLS As used herein, the term "red blood cell" (RBC) refers to a living, enucleate red blood cell (i.e., a mature erythrocyte) of a vertebrate. Unless the context requires otherwise, the term "vehicle" should be considered as synonymous with a red blood cell. Preferably the red blood cell is a mammalian red blood cell, advantageously a human red blood cell. As used herein, the term "mammal" refers to a member of the class Mammalia including, but not limited to, a rodent, lagomorph, pig or primate. More preferably, the animal is selected from the group consisting of: mouse, rat, rabbit, sheep, goat, horse, cow, and pig. Most preferably, the mammal is a human. As used herein the term "introducing" includes but is not limited to the administration of a red blood cell and/or an agent into a vertebrate. As used herein in reference to administration of an agent to a vertebrate, the term "introducing" includes but is not limited to causing the agent to enter the circulatory system of the vertebrate by transfusion or to infusing an agent to a target site. It is contemplated that a hollow needle, such as a hypodermic needle or cannula, is inserted through the wall of a blood vessel (e.g., a vein or artery) and the red blood cell is either injected using applied pressure or allowed to diffuse or otherwise migrate into the blood vessel. It is understood that the diameter of the needle is sufficiently large and the pressure sufficiently light to avoid damage of the cell by shear forces. Preferably, introduction of a red blood cell into a vertebrate in a method of the invention is intra-arterial or intravenous. Methods of blood cell transfusion are well known in the art. As used herein, the term "red blood cell delivery vector" means a red blood cell that has been loaded, or is capable of being loaded, with one or more agent-MTS conjugate(s) according to the methods of the invention and can be used to deliver the agent to a vertebrate. The red blood cell delivery vector is typically made to release the agent at a site of interest in the vertebrate using ultrasound as described above. SENSITISATION AND PRE-SENSITISATION The agents (as agent-MTS conjugates) are loaded into a red blood cell, which may be sensitised. Such a sensitised red blood cell preferably is rendered more susceptible to disruption by exposure to a stimulus than an unsensitised red blood cell. The stimulus may include any energy source, for example, ultrasound. One or more sensitisation steps may therefore be employed to increase the sensitivity of the cells to ultrasound. Furthermore, in order to enhance the loading efficiency of an agent into the red blood cell, the red blood cell may be subject to a "pre-sensitisation" step. Although the purpose of the pre-sensitisation step is to enhance the loading of the agent, an increase in sensitivity to lysis (for example, ultrasound mediated lysis) may also be achieved. Where more than one sensitisation step is involved, additional sensitisation steps may be performed at any stage in the process after the pre-sensitisation step. Thus, a second sensitisation step may be carried out either after the pre-sensitisation step but prior to loading, or after loading. Further sensitisation steps may be performed as required. Generally, where present, the sensitisation steps and the loading step are temporally separated. For example, cells are typically allowed to rest in buffer, such as PBS/Mg/glucose buffer, for at least 30 mins, preferably at least 60 mins, after a pre- sensitisation step to allow the cells to recover prior to loading or further sensitisation steps. It may be desirable to allow cells to rest for several hours, such as overnight, after the loading step. However, where passive loading is used, the sensitisation step may be effectively carried out at the same time as the agent is being loaded. The pre-sensitisation step increases the efficiency of loading of an agent into a red blood cell, compared to a red blood cell which has not been subject to pre- sensitisation. The pre-sensitisation may take the form of an electrosensitisation step, as described below. Alternatively, or in addition, the pre-sensitisation may be effected by for example the use of ultrasound, electromagnetic radiation such as microwaves, radio waves, gamma rays and X-rays may be used. In addition, the use of chemical agents such as DMSO and pyrrolidinone may be envisaged. Furthermore, thermal energy may be imparted on the red blood cells to pre-sensitise them. This may be achieved by raising the temperature of the red blood cells by conventional means, by heat shock, or by the use of microwave irradiation. In general, any method which allows pores to be formed oh the surface membrane of a red blood cell is a suitable candidate for use as a pre-sensitisation step. Preferably, the sensitisation step comprises an electrosensitisation procedure as described below. We have found that the efficiency of sensitisation for given electrical parameters varies depending on the cell density and it may therefore be necessary to perform a titration of cell density and or electrical parameters to establish the optimum concentration. By way of guidance, we have found that cells sensitised at a density of about 6-8 10 s cells/ml have good sensitivity to ultrasound. It will be appreciated that pre-sensitisation of a red blood cell may enhance the efficiency of loading of an agent-MTS conjugate, even where passive loading is used. PRE-SENSITISATION USING ULTRASOUND Where a pre-sensitisation step is present, this typically involves electrosensitisation (described in detail below); however, as noted above, ultrasound may also be used to pre-sensitise red blood cells. Such use of ultrasound is also referred to herein as "sonoporation". Exposure of red blood cells to ultrasound is believed to result in non-destructive and transient membrane poration (Miller et al, 1998, Ultrasonics 36, 947-952). As used herein, the term "ultrasound" refers to a form of energy which consists of mechanical vibrations the frequencies of which are so high they are above the range of human hearing. The lower frequency limit of the ultrasonic spectrum may generally be taken as about 20 kHz. Most diagnostic applications of ultrasound employ frequencies in the range 1 and 15 MHz (from Ultrasonics in Clinical Diagnosis. Edited by PNT Wells, 2nd. Edition, Publ. Churchill Livingstone [Edinburgh, London & NY, 1977]. Ultrasound has been used in both diagnostic and therapeutic applications. When used as a diagnostic tool ("diagnostic ultrasound"), ultrasound is typically used in an energy density range of up to about 100 mW/cm 2 (FDA recommendation), although energy densities of up to 750mW/cm have been used. In physiotherapy, ultrasound is typically used as an energy source in a range up to about 3 to 4 W/cm " (WHO recommendation). In other therapeutic applications, higher intensities of ultrasound may be employed, for example, HIFU at 100 /cm 2 up to lkW/cm 2 (or even higher) for short periods of time. The term "ultrasound" as used in this specification is intended to encompass diagnostic, therapeutic and focused ultrasound. Focused ultrasound (FUS) allows thermal energy to be delivered without an invasive probe (see Morocz et al, 1998 Journal of Magnetic Resonance Imaging Vol.8, No.l, pp.136-142. Another form of focused ultrasound is high intensity focused ultrasound (HIFU) which is reviewed by Moussatov et al. in Ultrasonics, 1998 Vol.36, No.8, pp.893-900 and TranHuuHue et al. in Acustica, 1997, Vol.83, No.6, pp.1103-1106. Preferably, the red blood cells are pre-sensitised by exposure to ultrasound that has an energy density in the therapeutic range. In a highly preferred embodiment, treatment is at 2.5W/cm 2 for 5 min using a 1MHz ultrasound head. This combination is however not intended to be limiting. Indeed, various combinations of frequency, energy density and exposure time may be used to pre-sensitise the red blood cells so that their loading efficiency is increased. LOADING As used herein, the term "loading" refers to introducing into a red blood at least one agent, and the term "loaded" is to be construed accordingly. Preferably, the agent is internalised into the red blood cell. As described above, the agents to be delivered according to the invention are provided as agent-MTS conjugates, which may be loaded into red blood cells by any suitable means. Passive loading means, where the agent-MTS conjugates cross the red blood cell membrane and "self-load" into the red blood cell vehicle are included, as well as "active" loading, such as by hypotonic dialysis. Furthermore, the agent-MTS conjugates may be expressed as fusion proteins comprising an agent to be delivered, together with an MTS. The fusion protein may be expressed from a transgene encoding the fusion protein, as described in further detail below. In passive loading, a source of red blood cells is provided. The red blood cells are then exposed to agent-MTS conjugate under conditions which allow the translocation of the agent-MTS conjugate into the red blood cell. The red blood cells are exposed for a sufficient amount of time to allow a suitable loading level to be achieved. Progress of loading may be monitored by any suitable means. Passive loading may be aided by the concurrent, prior or post- application of an active loading method, as described in detail below. Loading of a red blood cell with more than one agent may be performed such that the agents are loaded individually (in sequence) or together (simultaneously or concurrently). Such co-loading may involve any combination of agent-MTS conjugates. Loading is generally performed in a separate procedure to the "sensitising" procedure. The agents may be first admixed at the time of contact with the red blood cells or prior to that time. Where a pre-sensitisation step is undertaken, the red blood cells may be loaded either after the pre-sensitisation procedure or after one or more sensitisation procedures, preferably after the cells have rested. In this embodiment, the loading may be performed by any desired technique. Thus, a pre-sensitised and loaded cell may be sensitised. Furthermore, a pre-sensitised and subsequently sensitised cell may be loaded. The loading may be performed by a procedure selected from the group consisting of electroporation, iontophoresis, sonoporation, microinjection, calcium precipitation, membrane intercalation, microparticle bombardment, lipid-mediated transfection, viral infection, osmosis, osmotic pulsing, osmotic shock, diffusion, endocytosis, mechanical perforation/restoration of the plasma membrane by shearing, single-cell injection or a combination thereof. These are referred to here as "active" loading means. Sonoporation as a method for loading an agent into a cell is disclosed in, for example, Miller et al (1998), Ultrasonics 36, 947-952. Iontophoresis uses electrical current to activate and to modulate the diffusion of a charged molecule across a biological membrane, such as the skin, in a manner similar to passive diffusion under a concentration gradient, but at a facilitated rate. In general, iontophoresis technology uses an electrical potential or current across a semipermeable barrier. By way of example, delivery of heparin molecules to patients has been shown using iontophoresis, a technique which uses low current (d.c.) to drive charged species into the arterial wall. The iontophoresis technology and references relating thereto is disclosed in WO 97/49450. In a highly preferred embodiment, the red blood cell is pre-sensitised by electrosensitisation, and loaded using osmotic shock. If more than one agent is employed, the same or a different technique may be used to load the second agent into the red blood cell. Preferably the red blood cells of the present invention are pre- sensitised, sensitised and loaded in vitro or ex-vivo. Preferably loading is carried out by an osmotic shock procedure. The term "osmotic shock" is intended herein to be synonymous with the term "hypotonic dialysis" or "hypoosmotic dialysis". A preferred osmotic shock/hypotonic dialysis method is based on the method described in Eichler et al, 1986, Res. Exp. Med. 186: 407-412. This preferred method is as follows. Washed red blood cells are suspended in 1 ml of PBS (150 mM NaCl, 5 mM K HPO 4 /KH 2 PO 4 ; pH 7.4) to obtain a hematocrit of approximately 60%. The suspension is placed in dialysis tubing (molecular weight cut-off 12-14,000; Spectra- Por; prepared as outlined below) and swelling of cells obtained by dialysis against 100 ml of 5 mM K 2 HPO 4 /KH 2 PO 4 , pH 7.4 for 90 minutes at 4°C. Resealing is achieved by subsequent dialysis for 15 minutes at 37°C against 100ml of PBS containing 10 mM glucose. Check and adjust pH withlM NaOH). Cells are then washed in ice cold PBS containing 10 mM glucose using centrifugation. An alternative method of resealing involves dialysis for 60 minutes at 37°C against 100ml of Bax-modified buffer (mBAX: PH 7.4; 2.68mM KC1, 1.47M KH 2 P0 4ι 136mM NaCl, 8.1mM Na 2 HPO 4> 5mM glucose, 5mM adenine, 5mM MgCl 2. Check and adjust pH with 1M NaOH) or Bax buffer (BAX: PH 7.4; , 2.68mM KC1, 1.47M KH 2 PO 4 , 136mM NaCl, 8.1mM Na 2 HPO 4, 5mM glucose, 5mM adenosine, 5mM MgCl 2 . Other osmotic shock procedures include the method described in U.S. Pat. No. 4,478,824. That method involves incubating a packed red blood cell fraction in a solution containing a compound (such as dimethyl sulphoxide (DMSO) or glycerol) which readily diffuses into and out of cells, rapidly creating a transmembrane osmotic gradient by diluting the suspension of red blood cell in the solution with a near- isotonic aqueous medium. This medium contains an anionic agent to be introduced (such as inosine monophosphate or a phosphorylated inositol, for example inositol hexaphosphate) which may be an allosteric effector of haemoglobin, thereby causing diffusion of water into the cells with consequent swelling thereof and increase in permeability of the outer membranes, of the cells. This increase in permeability is maintained for a period of time sufficient only to permit transport of the anionic agent into the cells and diffusion of the readily-diffusing compound out of the cells. This method is of limited effectiveness where the desired agent to be loaded into cells is not anionic, or is anionic or polyanionic but is not present in the near-isotonic aqueous medium in sufficient concentration to cause the needed increase in cell permeability without cell destruction. U.S. Patent No. 4,931,276 and WO 91/16080 also disclose methods of loading red blood cells with selected agents using an osmotic shock technique. Therefore, these techniques can be used to enable loading of red blood cells in the present invention. Effective agents which may advantageously be loaded into red blood cells using the modified method provided in U.S. Patent No. 4,931,276 include peptides, purine analogues, pyrimidine analogues, chemotherapeutic agents and antibiotic agents. These agents frequently present drug delivery problems. Specific compounds include but are not limited to tryptophan, phenylalanine and other water-soluble amino acid compounds. Several derivatives of the unnatural analogues of the nucleic acid bases adenine, guanine, cytosine and thymine are well known as useful therapeutic agents, e.g. 6-mercaptopurine (6MP) and azathioprine, which are commonly used as immunosuppressants and inhibitors of malignant cell growth, and azidothymidine (AZT) and analogues thereof which are useful as anti- viral agents, particularly in the treatment of AIDS. It has been shown that the action of these unnatural base derivatives is dependent on intra-cellular conversion thereof to phosphorylated forms (Chan et al, 1987, Pharmacotherapy, 7: 165;14 177; also Mitsuya et al, 1986, Proc. Natl. Acad. Sci. U.S.A., 83: 1911-1915). An alternative osmotic shock procedure is described in U.S. Patent No. 4,931,276 which is incorporated herein by reference. Alternatively, loading may be carried out by a microparticle bombardment procedure. Microparticle bombardment entails coating gold particles with the agent to be loaded, dusting the particles onto a 22 calibre bullet, and firing the bullet into a restraining shield made of a bullet-proof material and having a hole smaller than the diameter of the bullet, such that the gold particles continue in motion toward cells in vitro and, upon contacting these cells, perforate them and deliver the payload to the cell cytoplasm. It will be appreciated by one skilled in the art that combinations of methods may be used to facilitate the loading of a red blood cell with agents of interest according to the invention. Likewise, it will be appreciated that a first and second agent, may be loaded concurrently or sequentially, in either order, into a red blood cell in any method of the present invention. As is apparent to one of skill in the art, any one or more of the above techniques can be used to load red blood cells for use in the invention, either prior to, simultaneously with, separate from or in sequence to the sensitisation procedure. For example, U.S. Patent No. 4,224,313 discloses a process for preparing a mass of loaded cells suspended in a solution by increasing the permeability of the cell membranes by osmotic pressure or an electric field, or both, loading agents by passage from a solution through the membranes of increased permeability, restoring the original permeability by sealing the membranes by regeneration effect, and separating the cells from the solution in which they are suspended. In that procedure, the agents in solution which are to be loaded include i) a pharmaceutical substance which reacts chemically or physically with substances in the extracellular milieu and which, when loaded into the cell, would prematurely destroy the cell membranes, and ii) at least one blood- compatible sugar and protein capable of providing hydrogen bridge bonding- or of entering into covalent bonds with the pharmaceutical substance, thereby inhibiting the reaction of the pharmaceutical substance with the cell membranes. It will be appreciated by one skilled in the art that combinations of methods may be used to facilitate the loading of a red blood cell with agents of interest according to the invention. Likewise, it will be appreciated that a first and second agent, may be loaded concurrently or sequentially, in either order, into a red blood cell in any method of the present invention. The concentration of agent used in the loading procedure may need to be optimised. Preferably loading takes place over a period of at least 30 mins, more preferably about 90 mins. In an alternative embodiment of the invention, a transgenic animal is employed to produce red blood cells pre-loaded with agent-MTS conjugates. This embodiment involves expression of a transgene encoding an polypeptide agent-MTS conjugate in a transgenic animal and is described in further detail below. ELECTROSENSITISATION The red blood cell vehicles of the present invention may be sensitised to ultrasound or other sources of energy by the use of an electric field ("electrosensitisation"). Electrosensitisation may also be used as a means of pre- sensitising red blood cells. The term "electrosensitisation" encompasses the destabilisation of cells without causing fatal damage to the cells. According to this method, a momentary exposure of a cell to one or more pulses at high electric field strength results in membrane destabilisation. The strength of the electric field is adjusted up or down depending upon the resilience or fragility, respectively, of the cells being loaded and the ionic strength of the medium in which the cells are suspended. Electrosensitisation typically occurs in the absence of the agent to be loaded into the cell. Electroporation, which facilitates passage of agents into the cell, occurs in the present of an exogenous agent to be loaded, and is well known in the art. Electroporation has been used in both in vitro and in vivo procedures to introduce foreign material into living cells. With in vitro applications, a sample of live cells is first mixed with the agent of interest and placed between electrodes such as parallel plates. Then, the electrodes apply an electrical field to the cell/implant mixture. Examples of systems that perform in vitro electroporation include the Electro Cell Manipulator ECM600 product, and the Electro Square Porator T820, both supplied by the BTX Division of Genetronics, Inc (see US Patent No 5,869,326). These known electroporation techniques (both in vitro and in vivo) function by applying a brief high voltage pulse to electrodes positioned around the treatment region. The electric field generated between the electrodes causes the cell membranes to temporarily become porous, whereupon molecules of the agent of interest enter the cells. In known electroporation applications, this electric field comprises a single square wave pulse on the order of IkV/cm, of about 100 μs duration. Such a pulse may be generated, for example, in known applications of the Electro Square Porator T820. Electrosensitisation may be performed in a manner substantially identical to the procedure followed for electroporation, with the exception that the electric field is delivered in the absence of an exogenous agent of interest, as set forth below, and may be carried out at different electric field strengths (and other parameters) from those required for electroporation. For example, lower field strengths may be used for electrosensitisation. Preferably, the electric field has a strength of from about 0.1 kV /cm to about 10 kV/cm under in vitro conditions, more preferably from about 1.5 kV/cm to about 4.0 kV/cm under in vitro conditions. Most preferably, the electric field strength is about 3.625kV/cm under in vitro conditions. Preferably the electric field has a strength of from about 0.1 kV/cm to about 10 kV/cm under in vivo conditions (see WO97/49450). More preferably, the electric field strength is about 3.625kV/cm under in vitro conditions. Preferably the application of the electric field is in the form of multiple pulses such as double pulses of the same strength and capacitance or sequential pulses of varying strength and/or capacitance. A preferred type of sequential pulsing comprises delivering a pulse of less than 1.5 kV/cm and a capacitance of greater than 5 μF, followed by a pulse of greater than 2.5 kV/cm and a capacitance of less than 2 μF, followed by another pulse of less than 1.5 kV/cm and a capacitance of greater than 5 μF. A particular example is 0.75 kV/cm, 10 μF; 3.625 kV/cm, 1 μF and 0.75 kV/cm, 10 μF. Preferably the electric pulse is delivered as a waveform selected from an exponential wave form, a square wave form and a modulated wave form. As used herein, the term "electric pulse" includes one or more pulses at variable capacitance and voltage and including exponential and/or square wave and/or modulated wave forms. Other electroporation procedures and methods employing electroporation devices are widely used in cell culture, and appropriate instrumentation, including the use of flow cell technology, is well known in the art. These procedures and methods may be adapted to perform electrosensitisation on a red blood cell. In a particularly preferred embodiment, the following electrosensitisation protocol is used. Cells are suspended in PBS to yield concentrations of about 6-8x10 cells/ml and 0.8 ml aliquots are dispensed into sterile electroporation cuvettes (0.4 cm electrode gap) and retained on ice for 10 min. Cells are then exposed to an sensitisation strategy involving delivery of two electric pulses (field strength = 3.625 kV/cm at a capacitance of 1 μF) using a BioRad Gene Pulser apparatus. Cells are immediately washed with PBS containing MgCl (4mM) (PBS/Mg) and retained at room temperature for at least 3 Omin in the PBS/Mg buffer at a concentration of 7x10 8 cells/ml to facilitate re-sealing. Optionally, cells are subsequently washed and suspended at a concentration of 7x10 cells/ml in PBS/Mg containing 10 mM glucose (PBS/Mg/glucose) for at least 1 hour. SELECTIVE RELEASE USING ULTRASOUND The agents which are loaded into a red blood cell may be released from the red blood cells and into their surroundings, in this case at or into the target site, tissue or cell, by the application of ultrasound directed at a target site, tissue and/or cell. Furthermore, the agent may be delivered to the target site by application of ultrasound to vessels, for example, blood vessels, feeding the target site. A general discussion on ultrasound, including different types of ultrasound (for example, diagnostic, therapeutic and focussed ultrasound), is presented above. Preferably, a combination of diagnostic ultrasound and a therapeutic ultrasound is employed to effect selective release. This combination is not intended to be limiting, however, and the skilled reader will appreciate that any variety of combinations of ultrasound may be used. Additionally, the energy density, frequency of ultrasound, and period of exposure may be varied. What is important is that the application of ultrasound is able to selectively disrupt the sensitised red blood cells to effect release of agent, without substantially disrupting or damaging endogenous red blood cells. Preferably the ultrasound is applied to a target cell or target tissue with sufficient strength to disrupt loaded and sensitised red blood cells but without damaging the target tissue or surrounding tissues. In this context, the term "damage or damaging" does not include a transient permeabilisation of the target site by the ultrasound energy source. Such a permeabilisation may facilitate uptake of the released payload at the target site. Preferably the exposure to an ultrasound energy source is at a power density of from about 0.05 to about 100 Wcm "2 . Even more preferably, the exposure to an ultrasound energy source is at a power density of from about 1 to about 15 Wcm "2 . Preferably the exposure to an ultrasound energy source is at a frequency of from about 0.015 to about 10.0 MHz. More preferably the exposure to an ultrasound energy source is at a frequency of from about 0.02 to about 6.0 MHz. Preferably the exposure is for periods of from about 10 milliseconds to about 60 minutes. More preferably the exposure is for periods of from about 1 second to about 5 minutes. Depending on the amount of agent which it is desired to release, however, the exposure may be for a longer duration, for example, for 15 minutes. Particularly preferably the patient is exposed to an ultrasound energy source at an acoustic power density of from about 0.05 Wcm " to about 10 Wcm " with a frequency ranging from about 0.015 to about 10 MHz (see WO 98/52609). However, alternatives are also possible, for example, exposure to an ultrasound energy source at an acoustic power density of above 100 Wcm "2 , but for reduced periods of time, for example, lOOOWcm "2 for periods in the millisecond range or less. Use of ultrasound is advantageous as, like light, it can be focused accurately on a target. Moreover, ultrasound is advantageous as it can be focussed more deeply into tissues unlike light. It is therefore better suited to whole-tissue penetration (such as but not limited to a lobe of the liver) or whole organ (such as but not limited to the entire liver or an entire muscle, such as the heart) delivery of agents according to the present invention. In addition, ultrasound may induce a transient permeabilisation of the target site so that uptake of a released payload is facilitated at the target site. Another important advantage is that ultrasound is a non-invasive stimulus which is used in a wide variety of diagnostic and therapeutic applications. By way of example, ultrasound is well known in medical imaging techniques and, additionally, in orthopaedic therapy. Furthermore, instruments suitable for the application of ultrasound to a subject vertebrate are widely available and their use is well known in the art. In methods of the invention, release of the agent is effected by exposure of red blood cells either in vitro or ex-vivo to an effective amount of a diagnostic ultrasound energy source or a therapeutic ultrasound energy source as described in US Patent No. 5558092 and WO94/28873. The agent, which is released from a red blood cell for use in the present invention may be referred to as the "payload" of that cell. Preferably the agent is released from the red blood cell by treatment of a target site, tissue or cell with ultrasound. The selective release of the agent at the target site can be determined by observing a) the amount which has been released at the target site, tissue or cell and b) its effect on the target site, tissue or cell, the latter determining whether its delivery should increase, decrease or be discontinued. AGENTSAND DELIVERY OFAGENTS The method of the present invention is useful for the delivery of agents to a selected site in a vertebrate body, whether an organ, part of an organ or otherwise, in the presence or absence of specific targeting means. This is achieved, as set out above, by the sele ' ctive disruption by ultrasound at the selected target site of preferably electrosensitised red blood cells loaded with the agent of choice. The agents to be delivered according to our invention are agent-MTS conjugates comprising membrane translocation sequences. Such agents are able to cross the cell membrane and enter the intracellular environment of a target cell. Agents useful for use in the present invention are set out below. Preferred agents include those useful for imaging of tissues in vivo or ex vivo. For example, imaging agents, such as antibodies which are specific for defined molecules, tissues or cells in an organism, may be used to image specific parts of the body by releasing them at a desired location using ultrasound. This allows imaging agents which are not completely specific for the desired target, and which might otherwise lead to more general imaging throughout the organism, to be used to image defined tissues or structures. For example, an antibody which is capable of imaging endothelial tissue may be used to image endothelial cells in lower body vasculature, for example, lower limbs, by releasing the antibody selectively in the lower body by applying ultrasound thereto. As used herein, the term "agent" includes but is not limited to an atom or molecule, wherein a molecule may be inorganic or organic, a biological effector molecule and/or a nucleic acid encoding an agent such as a biological effector molecule, a protein, a polypeptide, a peptide, a nucleic acid, an oligonucleotide, a peptide nucleic acid (PNA), a virus-like particle, a nucleotide, a ribonucleotide, a synthetic analogue of a nucleotide, a synthetic analogue of a ribonucleotide, a modified nucleotide, a modified ribonucleotide, an amino acid, an amino acid analogue, a modified amino acid, a modified amino acid analogue, a steroid, a proteoglycan, a lipid, a fatty acid and a carbohydrate. An agent may be in solution or in suspension (e.g., in crystalline, colloidal or other particulate form). The agent may be in the form of a monomer, dimer, oligomer, etc, or otherwise in a complex. The agent may be coated with one or more molecules, preferably macromoleucles, most preferably polymers such as PEG (polyethylene glycol). Use of a PEGylated agent increases the circulating lifetime of the agent once released. The agent may be an imaging agent, by which term is meant an agent which may be detected, whether in vitro in the context of a tissue, organ or organism in which the agent is located. The imaging agent may emit a detectable signal, such as light or other electromagnetic radiation. The imaging agent may be a radio-isotope as known in the art, for example P or S or Tc, or a molecule such as a nucleic acid, polypeptide, or other molecule as explained below conjugated with such a radio- isotope. The imaging agent may be opaque to radiation, such as X-ray radiation. The imaging agent may also comprise a targeting means by which it is directed to a particular cell, tissue, organ or other compartment within the body of an animal. For example, the agent may comprise a radiolabelled antibody specific for defined molecules, tissues or cells in an organism. The imaging agent may be combined with, conjugated to, mixed with or combined with, any of the agents disclosed herein. It will be appreciated that it is not necessary for a single agent to be used, and that it is possible to load two or more agents for into the vehicle. Accordingly, the term "agent" also includes mixtures, fusions, combinations and conjugates, of atoms, molecules etc as disclosed herein. For example, an agent may include but is not limited to: a nucleic acid combined with a polypeptide; two or more polypeptides conjugated to each other; a protein conjugated to a biologically active molecule (which may be a small molecule such as a prodrug); or a combination of a biologically active molecule with an imaging agent. As used herein, the term "biological effector molecule" or "biologically active molecule" refers to an agent that has activity in a biological system, including, but not limited to, a protein, polypeptide or peptide including, but not limited to, a structural protein, an enzyme, a cytokine (such as an interferon and/or an interleukin) an antibiotic, a polyclonal or monoclonal antibody, or an effective part thereof, such as an Fv fragment, which antibody or part thereof may be natural, synthetic or humanised, a peptide hormone, a receptor, and a signalling molecule. Included within the term "immunoglόbulin" are intact immunoglobulins as well as antibody fragments such as Fv, a single chain Fv (scFv), a Fab or a F(ab') . Preferred immunoglobulins, antibodies, Fv fragments, etc are those which are capable of binding to antigens in an intracellular environment, known as "intrabodies" or "intracellular antibdoies". An "intracellular antibody" or an "intrabody" is an antibody which is capable of binding to its target or cognate antigen within the environment of a cell, or in an environment which mimics an environment within the cell. Selection methods for directly identifying such "intrabodies" have been proposed, such as an in vivo two-hybrid system for selecting antibodies with binding capability inside mammalian cells. Such methods are described in International Patent Application number PCT/GB00/00876, hereby incorporated by reference. Techniques for producing intracellular antibodies, such as anti-β-galactosidase scFvs, have also been described in Martineau, et al, 1998, JMol Biol 280, 117-127 and Visintin, et al, 1999, Proc. Natl. Acad. Set USA 96, 11723-11728. An agent may include a nucleic acid, as defined below, including, but not limited to, an oligonucleotide or modified oligonucleotide, an antisense oligonucleotide or modified antisense oligonucleotide, cDNA, genomic DNA, an artificial or natural chromosome (e.g. a yeast artificial chromosome) or a part thereof, RNA, including mRNA, tRNA, rRNA or a ribozyme, or a peptide nucleic acid (PNA); virus-like particles; a nucleotide or ribonucleotide or synthetic analogue thereof, which may be modified or unmodified; an amino acid or analogue thereof, which may be modified or unmodified; a non-peptide (e.g., steroid) hormone; a proteoglycan; a lipid; or a carbohydrate. If the biological effector molecule is a polypeptide, it may be loaded directly into a red blood cell of the invention; alternatively, a nucleic acid molecule bearing a sequence encoding the polypeptide, which sequence is operatively linked to transcriptional and translational regulatory elements active in a cell at the target site, may be loaded. Small molecules, including inorganic and organic chemicals, are also of use in the present invention. In a particularly preferred embodiment of the invention, " the biologically active molecule is a pharmaceutically active agent, for example, an isotope. A preferred embodiment of the invention comprises loading a ribozyme or an oligonucleotide such as an antisense oligonucleotide comprising a membrane translocation sequence into a red blood cell, which is optionally sensitised, for delivery into a target cell or tissue. Particularly useful classes of biological effector molecules include, but are not limited to, antibiotics, anti-inflammatory drugs, angiogenic or vasoactive agents, growth factors and cytotoxic agents (e.g., tumour suppressers). Cytotoxic agents of use in the invention include, but are not limited to, diptheria toxin, Pseudomonas exotoxin, cholera toxin, pertussis toxin, and the prodrugs peptidyl-p-phenylenediamine-mustard, benzoic acid mustard glutamates, ganciclovir, 6-methoxypurine arabinonucleoside (araM), 5-fluorocytosine, glucose, hypoxanthine, methotrexate-alanine, N-[4-(a-D- galactopyranosyl) benyloxycarbonylj-daunorubicin, amygdalin, azobenzene mustards, glutamyl p-phenylenediamine mustard, phenolmustard-glucuronide, epirubicin- glucuronide, vinca-cephalosporin,phenylenediamine mustard-cephalosporin, nitrogen- mustard-cephalosporin, phenolmustard phosphate, doxorubicin phosphate, mitomycin phosphate, etoposide phosphate, palytoxin-4-hydroxyphenyl-acetamide, doxorubicin- phenoxyacetamide, melphalan-phenoxyacetamide, cyclophosphamide, ifosfamide or analogues thereof. If a prodrug is loaded in inactive form, a second biological effector molecule may be loaded into the red blood cell of the present invention. Such a second biological effector molecule is usefully an activating polypeptide which converts the inactive prodrug to active drug form, and which activating polypeptide is selected from the group that includes, but is not limited to, viral thymidine kinase (encoded by Genbank Accession No. J02224), carboxypeptidase A (encoded by Genbank Accession No. M27717), α-galactosidase (encoded by Genbank Accession No. M13571), β-glucuronidase (encoded by Genbank Accession No. M15182), alkaline phosphatase (encoded by Genbank Accession No. J03252 J03512), or cytochrome P- 450 (encoded by Genbank Accession No. D00003 N00003), plasmin, carboxypeptidase G2, cytosine deaminase, glucose oxidase, xanthine oxidase, β- glucosidase, azoreductase, t-gutamyl transferase, β-lactamase, or penicillin amidase. Either the polypeptide or the gene encoding it may be loaded; if the latter, both the prodrug and the activating polypeptide may be encoded by genes on the same recombinant nucleic acid construct. Furthermore, either the prodrug or the activator of the prodrug may be transgenically expressed and already loaded into the red blood cell according to the invention. The relevant activator or prodrug (as the case may be) is then loaded as a second agent according to the methods described here. Preferably the biological effector molecule is selected from the group consisting of a protein, a polypeptide, a peptide, a nucleic acid, an oligonucleotide, a peptide nucleic acid (PNA), a virus-like particle, a nucleotide, a ribonucleotide, a synthetic analogue of a nucleotide, a synthetic analogue of a ribonucleotide, a modified nucleotide, a modified ribonucleotide, an amino acid, an amino acid analogue, a modified amino acid, a modified amino acid analogue, a steroid, a proteoglycan, a lipid and a carbohydrate or a combination thereof (e.g., chromosomal material comprising both protein and DNA components or a pair or set of effectors, wherein one or more convert another to active form, for example catalytically). POLYMER THERAPEUTICS The agents may further be delivered attached to polymers, so long as either or both the agent and the polymer comprises a membrane translocation sequence. Polymer based therapeutics have been proposed to be effective delivery systems, and generally comprise one or more agents to be delivered attached to a polymeric molecule, which acts as a carrier. The agents are thus disposed on the polymer backbone, and are carried into the target cell together with the polymer. The agents may be coupled, fused, mixed, combined, or otherwise joined to a polymer. The coupling, etc between the agent and the polymer may be permanent or transient, and may involve covalent or non-covalent interactions (including ionic interactions, hydrophobic forces, Van der Waals interactions, etc). The exact mode of coupling is not important, so long as the agent is taken into a target cell substantially together with the polymer. For simplicity, the entity comprising the agent attached to the polymer carrier is referred to here as a "polymer-agent conjugate". Any suitable polymer, for example, a natural or synthetic polymer, may be used, preferably the carrier polymer is a synthetic polymer such as PEG. More preferably, the carrier polymer is a biologically inert molecule. Particular examples of polymers include polyethylene glycol (PEG), N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers, polyamidoamine (PAMAM) dendrimers, HEMA, linear polyamidoamine polymers etc. Any suitable linker for attaching the agent to the polymer may be used. Preferably, the linker is a biodegradable linker. Use of biodegradable linkers enables controlled release of the agent on exposure to the extracellular or intracellular environment. High molecular weight macromolecules are unable to diffuse passively into cells, and are instead engulfed as membrane-encircled vesicles. Once inside the vesicle, intracellular enzymes may act on the polymer-agent conjugate to effect release of the agent. Controlled intracellular release circumvents the toxic side effects associated with many drugs. Furthermore, agents may be conjugated, attached etc by methods known in the art to any suitable polymer, and delivered. The agents may in particular comprise any of the molecules referred to as "second agents", such as polypeptides, nucleic acids, macromolecules, etc, as described in the section above. In particular, the agent may comprise a pro-drug as described elsewhere. The ability to choose the starting polymer enables the engineering of polymer- agent conjugates for desirable properties. The molecular weight of the polymer (and thus the polymer-agent conjugate), as well as its charge and hydrophobicity properties, may be precisely tailored. Advantages of using polymer-agent conjugates include economy of manufacture, stability (longer shelf life) and reduction of immunogencity and side effects. Furthermore, polymer-agent conjugates are especially useful for the targeting of tumour cells because of the enhanced permeability and retention (EPR) effect, in which growing tumours are more 'leaky' to circulating macromolecules and large particules, allowing them easy access to the interior of the tumour. Increased accumulation and low toxicity (typically 10-20%) of the toxicity of the free agent) are also observed. Use of hyperbranched dendrimers, for example, PAMAM dendrimers, is particularly advantageous in that they enable monodisperse compositions to be made and also flexibility of attachment sites (within the interior or the exterior of the dendrimer). The pH responsiveness of polymer-agent conjugates, for example, those conjugated to polyamindoamine polymers, may be tailored for particular intracellular environments. This enables the drug to be released only when the polymer therapeutic encounters a particular pH or range of pH, i.e., within a particular intracellular compartment. The polymer agent conjugates may further comprise a targeting means, such as an immunoglobulin or antibody, which directs the polymer-agent conjugate to certain tissues, organs or cells comprising a target, for example, a particular antigen. Other targeting means are described elsewhere in this document, and are also known in the art. Particular examples of polymer-agent conjugates include "Smancs", comprising a conjugate of styrene-co-maleic anhydride and the antitumour protein neocarzinostatin, and a conjugate of PEG (poly-ethylene glycol) with L-asparaginase for treatment of leukaemia; PK1 (a conjugate of a HPMA copolymer with the anticancer drug doxorubicin); PK2 (similar to PK1, but furthermore including a galactose group for targeting primary and secondary liver cancer); a conjugate of HPMA copolymer with the anticancer agent captothecin; a conjugate of HPMA copolymer with the anticancer agent paclitaxel; HPMA copolymer-platinate, etc. Any of these polymer-agent conjugates are suitable for co-loading into the transgenic cells of the present invention. MEMBRANE TRANSLOCATION SEQUENCES The present invention encompasses the use of polypeptide sequences or domains which are able to direct proteins, polypeptides, and other molecules across the cell membrane and into the cell. The use of fragments or variants of such sequences which comprise membrane translocational activity is also included, as are sequences, variants, fragments etc of polypeptides capable of directing localisation into subcellular compartments (such as the nucleus). Such sequences, and their fragments, are referred to here as "membrane translocation sequences" or MTS. The presence of such sequences facilitates the intake of agent into a cell, and thus enables efficient intracellular delivery of agent. As explained above, one or more of these sequences may be coupled, fused, conjugated or otherwise joined to the agent to be delivered in order to effect intracellular delivery of the agent-MTS conjugate. In a highly preferred embodiment of the invention, polypeptides for delivery are expressed as fusion proteins with one or more membrane translocation sequences. There appears to be no restriction on the type of molecule that can penetrate cell membranes when fused to protein translocation sequences. Therefore the method of our invention may be used for the in vivo intracellular delivery of a wide variety of agents. For example, Fawell et al. (1994), Proc. Natl. Acad. Sci. USA. 91, 664-668 demonstrate that fusions can enter tissues in vivo in mice. Pooga et al. (1998), Nat. Biotechnol. 16, 857-861 demonstrate that fusions can penetrate the blood-brain barrier in rats. Many different protein translocation sequences have now been identified that can penetrate the cell membrane (reviewed by Lindgren et al. (2000), Trends Pharma. Sci. 21, 99-103; Morris et al. (2000), Curr. Opin. Biotech. 11, 461-466; Hawiger (1999), Curr. Opin. Chem. Biol. 3, 89-94). As used here, the term 'translocation' refers to transfer of an agent across a membrane such that the agent is internalised within a cell. Preferred membrane translocation sequences include the whole sequence or subsequences of the HIV-1- trans-activating protein (Tat), Drosophila Antennapedia homeodomain protein (Antp- HD), Herpes Simplex- 1 virus VP22 protein (HSV-VP22), signal-sequence-based peptides, Transportan and Amphiphilic model peptide, among others. These membrane translocation sequences, as well as domains and sequences from them which are useful for in the present invention, are described in further detail below. HIV- 1 -trans-activating protein (Tat) The Human Immunodeficiency Virus trans-activating protein (Tat) is a 86-102 amino acid long protein involved in HIV replication. Exogenously added Tat protein can translocate through the plasma membrane to reach the nucleus, where it transactivates the viral genome. Intraperitoneal injection of a fusion protein consisting of β-galactosidase and Tat results in delivery of the biologically active fusion protein to all tissues in mice (Schwarze et al., (1999), Science 285, 1569-72). Methods of delivering molecules such as proteins and nucleic acids into the nucleus of cells using Tat or Tat-derived polypeptides are described in detail in US Patent Numbers 5652122, 5670617, 5674980, 5747641 and 5804604. Vives et al. (1997), J Biol. Chem. 272, 16010-7 identified a sequence of amino acids 48-60 (CGRKKRRQRRRPPQC) from Tat important for translocation, nuclear localisation and trans-activation of cellular genes. This core sequence also includes a nuclear localisation sequence and has been found to exhibit translocational activity. Accordingly, our invention encompasses the use of polypeptides comprising the entire HIV-Tat sequence as well as polypeptides comprising the core sequence for translocating an agent into a cell. It will however be appreciated that variations about the core sequence, such as shorter or longer fragments (such as for example 47-58), may also possess translocational activity, and that these sequences may also be usefully employed. To date, numerous Tat derived short membrane translocation domains and sequences have been identified that possess translocation activity; furthermore, translocation has been found to occur in various different cell types (Lindgren et al. (2000), Trends Pharma. Sci. 21, 99-103). Examples of fragments which possess translocational activity include amino acids 37-72 (Fawell et al, (1994), Proc. Natl. Acad. Sci. USA. 91, 664-668), 37-62 (Anderson et al, (1993), Biochem. Biophys. Res. Commun. 194, 876-884) and 49-58 (having the basic sequence RKKRRQRRR). Any of these fragments may be used alone or in combination with each other, and/or preferably with the core sequence, to enable translocation of an agent into a cell. Internalisation of Tat is though to occur by endocytosis (Frankel & Pabo (1988), Cell 55, 1189-1193). Co-administration of basic peptides such as protamine or Tat fragments (amino acids 38-58) has been found to stimulate Tat uptake into cells. Accordingly, the present invention also encompasses the use of these and other agents which stimulate uptake ("translocation enhancers") to enhance the delivery of an agent into a cell. Use of such translocation enhancers need not necessarily be restricted to enhancing translocation of Tat conjugates/fusions - our invention encompasses the use of such enhancers to enhance delivery of conjugates and/or fusions with other membrane translocation sequences (and/or fragments or domains of these), as described below. Thus, one or more translocation enhancers may be administered to the recipient before, after or at the same time as the loaded red blood cells are administered. Alternatively, the red blood cell may be loaded with the translocation enhancer(s) as well as the agent, preferably joined to a membrane translocation sequence, to be delivered. Disruption of the red blood cell at the point of delivery releases both the agent to be delivered and the translocation enhancer, thus stimulating uptake of the agent by the target cell or tissue, etc. Tat-derived polypeptides lacking the cysteine rich region (22-36) and the carboxyl terminal domain (73-86) have been found to be particularly effective in tranlocation. Absence of the cysteine rich region and the carboxy terminal domain prevents spurious trans-activiation and disulphide aggregation. In addition, the reduced size of the transport polypeptide minimises interference with the biological activity of the molecule being transported and increases uptake efficiency. Such polypeptides are used in the methods described in US Patent Numbers 5652122, 5670617, 5674980, 5747641 and 5804604. Accordingly, the present invention encompasses the use of such Tat-derived polypeptides lacking the carboxyl terminal domain and/or the cysteine rich region to improve the efficiency of translocation. Preferably, the Tat- derived polypeptide lacks amino acids 73-86 of the Tat protein or amino acids 73-86 of the Tat protein. More preferably, the membrane translocation sequence comprises a Tat-derived protein which lacks both domains. Drosophila Antennapedia homeodomain protein (Antp-HD) Agents may be conjugated or fused with all or part of the Drosophila Antennapedia homeodomain protein, preferably, the third helix of Antp-HD, which also has cell penetration properties (reviewed in Prochiantz (1999), Ann. N. Y. Acad. Sci. 886, 172-9). Cell intemalization of the third helix of Antp-HD appears to be receptor- and endocytosis-independent. Derossi et al. (1996), J Biol. Chem. 271, 18188-93 suggest that the translocation process involves direct interactions with membrane phospholipids. The region responsible for translocation in Antp-HD has been localised to amino acids 43-58 (third helix), a 16 amino acid long peptide rich in basic amino acids having the sequence RQIKIWFQNRRMKWKK (Derossi, et al, (1994), J Biol. Chem. 269, 10444-50). This peptide is known as Penetratin ® and has been used to direct biologically active substances to the cytoplasm and nucleus of cells in culture (Theodore, et al. (1995), J. Neurosci. 15, 7158-7167). Chimeric peptides less than 100 amino acids and oligonucleotides up to 55 nucleotides are capable of being internalised. Thoren et al. (2000) FEBS Lett. 6, 265-8 show that Penetratin ® traverses a lipid bilayer, further supporting the idea that cell intemalization of the third helix of Antp-HD is receptor- and endocytosis-independent. Our invention therefore encompasses the use of Antp-HD or fragments of Antp-HD (including preferably fragments comprising, more preferably consisting of, RQIKIWFQNRRMKWKK, i.e., Penetratin) for intracellular delivery of agents. Antp-HD and its fragments may be conjugated with proteins and nucleic acids by methods known in the art, for example as described in WO 99/11809. This document also describes sequences homologous to Antp-HD isolated from other organisms, including vertebrates, mammals and humans; homologues of Penetratin ® are also described in EP 485578. The present invention encompasses the use of these and other homologues and fragments of these for delivery of agents into cells. Truncated and modified forms of Antp-HD and Penetratin are described in WO 97/12912, UK 9825000.4 and UK 9902522.3. For example, truncated polypeptides of 15 and 7 amino acids such as RRMKWKK have been found to be active in translocation. Accordingly our invention encompasses the use of such truncated and modified forms of Antp-HD and its homologues. To improve intracellular delivery, Antp-HD and/or its fragments may be conjugated to peptide nucleic acid (PNA), as described by Nielsen et al. (1991) Science 254, 1497-1500. PNA is resistant to proteases and nucleases and is much more stable in cells than regular DNA. Pooga et al. (1998) Nat Biotechnol. 16, 857-861 show that a 21-mer PNA complementary to human galanin receptor mRNA, coupled to Antp-HD, is efficiently taken up into Bowes melanoma cells, thus suppressing the expression of galanin receptors. Our invention therefore includes the use of conjugates and/or fusions of agents, membrane translocation proteins (and/or fragments) and peptide nucleic acid. Herpes Simplex- 1 virus VP22 protein The VP22 tegument protein of herpes simplex virus also exhibits membrane translocation activity. Thus, VP22 protein expressed in a subpopulation of cells spreads to other cells in the population (Elliot and O'Hare, 1997, Cell ' 818, 223-33). Fusion proteins consisting of GFP (Elliott and O'Hare, 1999, Gene Ther 6, 149-51), thymidine kinase protein (Dilber et al., 1999, Gene Ther 6, 12-21) or p53 (Phelan et al, 1998, Nat Biotechnol 16, 440-3) with VP22 have been targeted to cells in this manner. HSV-VP22 has the amino acid sequence NAATATRGRSAASRPTERPRAPARSASRPRRPVE and agents may be conjugated or fused to this polypeptide (or fragments exhibiting translocation activity) for delivery into cells. As noted above, an important property of HSV-VP22 is that when applied to the surrounding medium, VP-22 is taken up by cells and accumulates in the nucleus. Thus, fusion proteins of HSV-VP22 conjugated to GFP (Elliott and O'Hare (1999), Gene Ther. 6, 149-51), thymidine kinase protein (Dilber et al. (1999), Gene Ther. 6, 12-21) and p53 (Phelan et al. (1998), Nat. Biotechnol. 16, 440-3) have been targeted to cells in this manner. The mechanism of transport is thought to be via a Golgi- independent pathway. Fusion proteins comprising HSV-VP22 (and sub-sequences) and a protein o ' f interest, and the transport of such fusions into a cell are described in US 6017735. Proteins capable of being transported by the methods described in US 6017735 include those involved in apoptosis, suicide proteins and therapeutic proteins. A feature of HS V-VP22 is that it binds to microtubules in cells as described in WO 98/42742. Therefore, fusions, conjugates, etc of HSV-VP22 (including its fragments) with agents may be delivered into cells to stabilise microtubules and retard or enhance cell growth. Variants of VP22 may be prepared in which the potency of this property is altered. Agents which enhance or inhibit microtubule polymerisation or de- polymerisation may be delivered to enhance or retard cell growth. Furthermore, HSV- VP22 fusions/conjugates may be employed where microtubule transport of an agent to a particular intracellular compartment or location is desired. Signal-Sequence-Based Peptides Signal sequences of peptides are recognised by acceptor proteins that aid in ■ addressing the pre-protein from the translation machinery to the membrane of appropriate intracellular organelles. The core hydrophobic region of a signal peptide sequence may be used as a carrier for cellular import of relevant segments or motifs of intracellular proteins (Lin et al, 1995, JBiol Chem 270, 14255-14258; Liu et al, 1996, Proc Natl Acad Sci USA, 93, 11819-11824). Synthetic membrane translocation domains and sequences containing such hydrophobic regions are able to translocate into cells. The hydrophobic region, also known as the h region, consists of 7-16 non- conserved amino acids, and has been identified in 126 signal peptides ranging in length from 18-21 amino acids (Prabhakaran, 1990, Biochem J, 269,691-696). Any of these sequences may be employed in the present invention. Signal sequence based translocators are thought to function by acting as a leader sequence ("leading edge") to carry peptides and proteins into cells (reviewed by Hawiger (1999), Curr. Opin. Cell. Biol. 3, 89-94). Use of signal peptides for delivery of biologically active molecules is disclosed in US Patent No.l 5,807,746. It is known that import of polypeptides comprising the signal sequence h- region does not require membrane caveolae (Torgerson et al. J. Immunol. 161, 6084- 6092) or endosomal uptake (Lin et al. (1995), J. Biol. Chem. 270, 14255-14258; Hawiger (1997), Curr. Opin. Immunol. 9, 189-194) but requires an intact plasma membrane (Lin et al. (1995), J Biol. Chem. 270, 14255-14258). Furthermore, the uptake mechanism is concentration- and temperature-dependent, independent of cell type and receptor. Signal sequence based peptides can translocate into a number of cell types that include five human cell types (monocytic, endothelial, T lymphocyte, fibroblast and erythroleukemia) and three murine lines. Accordingly, the invention encompasses the use of membrane translocation sequences, including signal sequence h-regions, conjugates, fusions, etc for intracellular delivery of agents. Membrane translocation sequences comprising signal sequence based peptides coupled to nuclear localisation sequences (NLSs) may also be utilised. Thus, for example, the MPS peptide (Signal-sequence-based peptide I) is a chimera of the hydrophobic terminal domain of the viral gp41 protein and the NLS from the SV40 large antigen (GALFLGWLGAAGSTMGAWSQPKKKRKV) (Morris et al. (1997), Nucleic Acids Res. 25, 2730-2736), and has been found to be active in membrane translocation. The peptide AAVALLPAVLLALLAP (Signal-sequence-based peptide II) is derived from the nuclear localisation signal of NF- B p50 (Lin et al. (1996), Proc. Natl. Acad. Sci. USA 93, 11819-11824) and USF2 (Frenkel et al. (1998), J. Immunol. 161, 2881-2887). A peptide having the sequence AAVLLPVLLAAP is derived from from the Grb2 SH2 domain (Rojas et al. (1998), Nat. Biotechnol. 16, 370-375) and VTVLALGALAGVGVG from the Integrin β 3 cytoplasmic domain (Liu et al. (1996), Proc. Natl. Acad. Sci. USA 93, 11819-11824). Peptides comprising membrane translocation sequence-nuclear localisation sequence have been shown to enter several cell types. Membrane translocation sequences derived from the hydrophobic regions of the signal sequences from Kaposi's sarcoma fibroblast growth factor 1 (K-FGF; Lin et al. 1995, J Biol. Chem. 271, 5305-5308) and human β integrin (Liu et al. 1996, Proc. Natl. Acad. Sci. USA 93, 11819-11824), the fusion sequence of HIV-1 gp4 (Morris et al, 1997, Nucleic Acid Res, 25, 2730-2736) and the signal sequence of the variable immunoglobulin light chain Ig(v) from Caiman crocodylus (Chaloin et al., 1997, Biochemistry 36, 11179-11187) conjugated to ΝLS peptides originating from nuclear transcription factor kB (ΝF-κB; Zhang et al., 1998, Proc Natl Acad Sci USA 95, 9184-9189), SV40 T-antigen (Chaloin et al., 1998, Biochem. Biophys. Res. Commun. 243, 601-608) or K-FGF (Lin et al, 1995, J Biol. Chem. 270, 14255-14258) may also be employed. Any of the peptides described above may be used alone or in combination, preferably in conjunction with nuclear localisation sequences, to deliver fused or conjugated agents into a cell. Transportan Agents for delivery may be conjugated or fused or joined with transportan. Transportan comprises a fusion between the neuropeptide galanin and the wasp venom peptide mastoparan. It is found to be localised in both the cytoplasm and nucleus (Pooga et al. (1998) FASEB J. 12, 67-77). Transportan comprises the sequence GWTLNSAGYLLKINLKALAALAKKIL. Transportan may be used as a carrier vector for hydrophilic macromolecules. Cell-penetrating ability is not restricted by cell type and seems to be a general feature of this membrane translocation domain. Cellular uptake is not inhibited by unlabeled transportan or galanin and shows no toxicity at concentrations of 20 μM or less. However, concentrations of 50 μM decrease GTPase activity (Pooga et al. (1998), Ann. New York Acad. Sci. 863, 45-453). The mechanism of cell penetration by transportan is not clear; however, it is known to be energy independent and that receptors and endocytosis are not involved. Deletion analogues of transportan have been prepared (Soomets et al. (2000), Biochim. Biophys. Acta. 1467, 165-176) to identify those regions of the sequence responsible for translocation. Deletion of six amino acids from the N-terminus of transportan does not impair cell penetration. Deletions at the C-terminus or in the middle of the protein decrease or abolish translocation activity. Accordingly, the invention includes the use of transportan, as well as deletions of transportan comprising translocation activity (preferably N-terminal deletions of 1, 2, 3, 4, 5 or 6 amino acids) in the delivery of agents into cells. The invention furthermore includes the use of novel short analogues disclosed by Lindgren et al., 2000, Bioconjug Chem 11(5):619-26 with similar translocation properties but with reduced undesired effects such as inhibition of GTPase activity. Amphiphilic Model Peptide Agents may be conjugated with amphiphilic model peptide. Amphiphilic model peptide is a synthetic 18-mer (KLALKLALKALKAALKLA) first synthesised by Oehlke et al. (1998), Biochim. Biophys. Acta. 1414, 127-139. Analogues that show less toxicity and higher uptake have been synthesised by Scheller et al. (1999^ ) J. Peptide Sci. 5, 185-194. The only essential structural requirement for amphiphilic model peptides is a length of four complete helical turns. The membrane translocation sequence crosses the plasma membranes of mast cells and endothelial cells by both energy-dependent and -independent mechanisms. The uptake behaviour shows analogy to several membrane translocation domain sequences including Antp-HD and Tat. While it is clear from the above that any of the membrane translocation sequences (including domains and/or sequences and/or fragments of these exhibiting membrane translocation activity) may be used for the purpose of delivery of an agent into a cell, it should also be appreciated that other variations are also possible. For example, variations such as mutations, (including point mutations, deletions, insertions, etc) of any of the sequences disclosed here may be employed, provided that some membrane translocation activity is retained. Furthermore, it will be clear that any homologues of the membrane translocation proteins identified above, for example, from other organisms (as well as variations), may also be used. Particular domains or sequences from proteins capable of translocation through the nuclear and/or plasma membranes may be identified by mutagenesis or deletion studies. Alternatively, synthetic or expressed peptides having candidate sequences may be linked to reporters and translocation assayed. For example, synthetic peptides may be conjugated to fluoroscein and translocation monitored by fluorescence microscopy by methods described in Vives et al. (1997), J Biol Chem 272, 16010-7. Alternatively, green fluorescent protein may be used as a reporter (Phelan et al., 1998, Nat Biotechnol 16, 440-3). The membrane translocation sequence may be linked to the agent to be delivered such that more than one agent can be delivered into a cell. The protein or fragment may contain components that facilitate the binding of multiple agents, for example drugs such as naturally occurring or synthetic amino acids. In this manner up to 32 different agents can be linked to the membrane translocation sequence and delivered. Such a method of using a membrane translocation sequence to facilitate the transfer of drugs is described in detail in WO 00/01417. Agents may be fused to membrane translocation sequences, including proteins or fragments, using a variety of methods. Using peptide synthesis, the membrane translocation sequence can be chemically synthesised and linked with any peptide sequence or chemical compound (Lewin et al. (2000), Nat. Biotechnol. 18, 410-414) using methods well known in the art. Peptides can also be chemically cross-linked to larger peptides and proteins (Fawell et al. (1994), Proc. Natl Acad. Sci. USA 91, 664- 668). Furthermore, fusion proteins comprising the polypeptide agent fused to a membrane translocation sequence may be expressed in any suitable host, for example, a bacterial host (Νagahara et al. (1998), Nat. Med. 4, 1449-1452). The cDΝA of interest (including sequences encoding the membrane translocation protein or fragment as well as the polypeptide agent of interest) may be cloned in-frame downstream of an N-terminal leader, for example, comprising a 6-Histidine tag. This enables purification of the expressed recombinant fusion proteins using methods known in the art. Advantageously, and as described above, polypeptides for delivery are expressed as fusion proteins with such sequences and/or fragments. Delivery of red blood cells containing the fusion protein, disruption and release in the vicinity of the target cell or tissue etc enables efficient intracellular delivery of agent into the target. The agent(s) may also be chemically coupled, either directly or indirectly, to the membrane translocation proteins, fragments, etc. The coupling may be permanent or transient, " and may involve covalent or non-covalent interactions. Coupling technologies are well known in the art. Direct linkage may be achieved by means of a functional group on the agent such as a hydroxyl, carboxy or amino group. Indirect linkage can occur through a linking moiety such as, but not limited to, one or more of bi-functional cross-linking agents, as known in the art. In this manner, a second agent comprising such fusion and/or conjugate, etc to be easily loaded into a transgenic red blood cell. In a highly preferred embodiment of the invention, the agent-MTS conjugate is one which does not elicit an immune response, or one which elicits a minimal immune response, when the agent-MTS conjugate is exposed to the donor animal. Preferably, the membrane translocation sequence does not elicit, or elicits a minimal, immune response. Thus, preferably, the membrane translocation sequence may be derived from a mammalian source, or is otherwise a mammalian homologue of a membrane translocation sequence as disclosed above. Preferably, therefore, in relation to a human recipient, the membrane translocation sequence comprises a human transportan, a human amphiphilic model peptide, or a human signal-sequence-based peptide. In other words, a signal sequence from any known human protein may be used as the basis for designing a suitable translocation sequence. In the alternative, the membrane translocation sequence may be a humanised membrane translocation sequence, the term being understood to mean a sequence in which one or more residues of a membrane translocation sequence are substituted with other residues to minimise an immune response when the agent-MTS conjugate is exposed to a human. EXPRESSION OF TRANSGENE As described above, the agent-MTS conjugate may comprise a fusion protein expressed from a trangene in RBCs. This aspect of the invention is described in more detail in our co-pending British Patent Application No. 0101469.5 (Attorney Reference: P9981GB). Preferably, the red blood cell is a mammalian red blood cell. In order to obtain high level expression of a transgene product in RBCs, the transgene is preferably driven by or operably linked to a promoter that is specific for the erythroid cell lineage, most preferably, in reticulocytes. Reticulocytes are immature RBCs which have extruded their nucleus, but retain a large amount of RNA, and thus display a grainy basophilic staining pattern in hematoxylin and eosin stained preparations. Circulating reticulocytes, which make up approximately 1% of circulating blood cells are transient blood cells; after leaving the bone marrow, reticulocytes retain their RNA and thus their protein synthetic ability for approximately 24 hours, before full maturation into essentially mRNA-free erythrocytes. During its life cycle in circulating blood, reticulocytes, by virtue of their RNA content, continue to produce haemoglobin and thus continue to translate mRNAs, endogenous or recombinant, derived from genes which possess erythrocyte- specific promoters. Therefore, the polypeptides described above, driven by the erythrocyte promoters described below, will be expressed in virtually all circulating RBCs by virtue of transgene synthesis in reticulocytes prior to their maturation to mature RBCs. Any promoter known to be active in cells of the erythrocytic lineage may be used to direct the expression of a polypeptide in the methods of the invention. However, examples of promoters that direct high level expression of erythroid-specific genes include the globin gene promoters. Haemoglobin is expressed in a tissue- specific manner in RBCs, where it accounts for about 95%) of total cellular protein. Globin gene promoters include those for the I, J (β globin), L, M and N globin genes. Particularly preferred among these is the human β globin promoter, which is most active in adults. ε, γ G , A δ AND β GLOBIN GENES Human β globin (also known as J globin) genes are found in a cluster on chromosome 11, comprising about 50 kb of DNA that also includes one embryonic gene encoding ε globin (also known as M globin), two fetal genes encoding K globins γ , γ A (also known as G and A globins), and two adult genes encoding δ and β globin (also known as L and J globin), in that order (Fritsch et al., 1980, Cell 19: 959-972). It has been found that DNA sequences both upstream and downstream of the β globin translation initiation site are involved in the regulation of β globin gene expression (Wright et al., 1984, Cell 38:263). In particular, a series of four DNAse I super hypersensitive sites (now referred to as the locus control region, or LCR) located about 50 kilobases upstream of the human β globin gene are extremely important in eliciting properly regulated β globin-locus expression (Tuan et al., 1985, Proc. Natl. Acad. Sci. U.S.A. 83:1359-1363; PCT Patent Application WO 8901517 by Grosveld; Behringer et al, 1989, Science 245:971-973; Enver et al, 1989, Proc. Natl. Acad. Sci. U.S.A. 86:7033-7037; Hanscombe et al, 1989, Genes Dev. 3:1572-1581; Van Assendelft et al, 1989, Cell 56:967-977; Grosveld et al, 1987, Cell 51 :975-985). Thus, in a highly preferred embodiment of the invention, the transgene is operably linked to or its expression is regulated from a globin LCR. Expression systems, including expression vectors, useful for erythroid expression are described in detail in US Patent No. 5,538,885 and GB 2251622. The vectors described in this document comprise a promoter, a DNA sequence which codes for a desired polypeptide and a dominant control region, and the present invention preferably utilises such vectors. Preferably, the dominant control region comprises a micro locus which comprises a 6.5 kb fragment obtained by ligating the fragments: 2.1 kb Xbal - Xbal; 1.9 kb Hindlll - Hindlll; 1.5 kb Kpnl - Bglll; and 1.1 kb partial Sad; from the β-globin gene. As used herein the term "dominant control region" (or "DCR") means a sequence of DNA capable of conferring upon a linked gene expression system the property of host cell-type restricted, integration site independent, copy number dependent expression when integrated into the genome of a host compatible with the dominant control region. The dominant control region retains this property when fully reconstituted within the chromosome of the host cell; and the ability to direct efficient host cell-type restricted expression is retained even when fully reformed in a heterologous background such as a different part of the homologous chromosome or even a different chromosome. A method for making a desired peptide in transgenic animals is described in US Patent No 5,627,268. A transgenic animal is engineered to comprise an artificial gene, which is controlled by globin locus control region (LCR) and which encodes a fusion protein. In the fusion protein, the desired peptide is linked via a cleavable peptide bond to a globin polypeptide. The erythrocytes of the transgenic animal express the fusion protein which is incorporated into hemoglobin produced by the host cell. The desired peptide can be obtained from a hemolysate of the red cells of the transgenic animals by cleavage of the linking bond and separation of the peptide away from globin portions. Production of recombinant haemoglobin is described in US Patent No 5,821,351. Other promoters useful in the method of the invention include the promoter of the Erythroid-specific GATA-1 transcription factor gene or a heterologous construct comprising the enhancer from the GATA-1 transcription factor gene (Grande et al, 1999, Blood 93 :3276). Other alternatives include but are not limited to the NF-E2 proximal IB promoter (Moroni et al. 2000, JBC 275: 10567) and the B19 p6 promoter with or without an erythrocyte-specific enhancer element (Kurpad et al, 1999, J. Hematother. Stem. Cell. Res. 8:585). The skilled person will appreciate that any suitable promoter may be used, so long as it directs expression of the desired polypeptide at an appropriate level in the red blood cell. TRANSGENIC ANIMALS A transgenic animal is a non-human animal containing at least one foreign gene, called a transgene, in its genetic material. Preferably, the transgene is contained in the animal's germ line such that it can be transmitted to the animal's offspring. In relation to the present invention, transgenic animals are useful for producing RBCs comprising polypeptides, in particular therapeutic polypeptides. A number of techniques ' may be used to introduce the transgene into an animal's genetic material, including, but not limited to, microinjection of the transgene into pronuclei of fertilized eggs and manipulation of embryonic stem cells (U.S. Pat. No. 4,873,191 by Wagner and Hoppe; Palmiter and Brinster, 1986, Ann. Rev. Genet. 20:465-499; French Patent Application 2593827 published Aug. 7, 1987). Transgenic animals may carry the transgene in all their cells or may be genetically mosaic. According to the method of conventional transgenesis, additional copies of normal or modified genes are injected into the male pronucleus of the zygote and become integrated into the genomic DNA of the recipient mouse. The transgene is transmitted in a Mendelian manner in established transgenic strains. Constructs useful for creating transgenic animals useful according to the invention comprise genes encoding therapeutic molecules, preferably under the control of nucleic acid sequences directing their expression in cells of the erythroid lineage. Alternatively, therapeutic molecules encoding constructs may be under the control of their native promoters, or inducibly regulated. A transgenic animal expressing one transgene " can be crossed to a second transgenic animal expressing second transgene such that their offspring will carry both transgenes. Although the majority of studies have involved transgenic mice, other species of transgenic animal have also been produced, such as rabbits, sheep, pigs (Hammer et al., 1985, Nature 315:680-683; Kumar, et al, U.S. 05922854; Seebach, et al., U.S. 06030833) and chickens (Salter et al, 1987, Virology 157:236-240). While the transgenic animals described in the present invention are not limited to swine, the description which follows details the methodology for transgene expression in larger animals, such as swine, but may be adapted for smaller animals as need requires. Transgenic animals are currently being developed to serve as bioreactors for the production of useful pharmaceutical compounds (Van Brunt, 1988, Bio/Technology 6:1149-1154; Wilmut et al., 1988, New Scientist (My 7 issue) pp. 56-59). Methods of expressing recombinant protein via transgenic livestock have an important theoretical advantage over protein production in recombinant bacteria and yeast; namely, the ability to produce large, complex proteins in which post- translational modifications, including glycosylation, phosphorylation, subunit assembly, etc. are critical for the activity of the molecule. In particular, the present invention includes, but is not limited to, recombinant swine RBCs expressing agent-MTS fusion polypeptides. RBCs containing the agent- MTS fusion polypeptide may be prepared by introducing a recombinant nucleic acid molecule which encodes said agent-MTS fusion polypeptide into a tissue, such as bone marrow cells, using known transformation techniques. These transformation techniques include transfection and infection by retroviruses carrying either a marker gene or a drug resistance gene. See for example, Current Protocols in Molecular Biology, Ausubel et al. eds., John Wiley and Sons, New York (1987) and Friedmann (1989) Science 244:1275-1281. A tissue containing a recombinant nucleic acid molecule of the present invention may then be reintroduced into an animal using reconstitution techniques (See for example, Dick et al. (1985) Cell 42:71). The recombinant constructs described here may be used to produce a transgenic animal by any method known in the art, including, but not limited to, microinjection, embryonic stem (ES) cell manipulation, electroporation, cell gun, transfection, transduction, retro viral infection, etc. Transgenic animals of the present invention can be produced by introducing transgenes into the germline of the animal, particularly into the genome of bone marrow cells, e.g. hematopoietic cells. Embryonal target cells at various developmental stages can be used to introduce the human transgene construct. As is generally understood in the art, different methods are used to introduce the transgene depending on the stage of development of the embryonal target cell. One technique for transgenically altering an animal is to microinject a recombinant nucleic acid molecule into the male pronucleus of a fertilized egg so as to cause 1 or more copies of the recombinant nucleic acid molecule to be retained in the cells of the developing animal. The recombinant nucleic acid molecule of interest is isolated in " a linear form with most of the sequences used for replication in bacteria removed. Linearization and removal of excess vector sequences results in a greater efficiency in production of transgenic mammals. See for example, Brinster et al. (1985) PNAS 82:4438-4442. In general, the zygote is the best target for micro-injection. In the swine, the male pronucleus reaches a size which allows reproducible injection of DNA solutions by standard microinjection techniques. Moreover, the use of zygotes as a target for gene transfer has a major advantage in that, in most cases, the injected DNA will be incorporated into the host genome before the first cleavage. Usually up to 40 percent of the animals developing from the injected eggs contain at least 1 copy of the recombinant nucleic acid molecule in their tissues. These transgenic animals will generally transmit the gene through the germ line to the next generation. The progeny of the transgenically manipulated embryos may be tested for the presence of the construct by Southern blot analysis of a segment of tissue. Typically, a small part of the tail is used for this purpose. " The stable integration of the recombinant nucleic acid molecule into the genome of transgenic embryos allows permanent transgenic mammal lines carrying the recombinant nucleic acid molecule to be established. Alternative methods for producing a mammal containing a recombinant nucleic acid molecule of the present invention include infection of fertilized eggs, embryo-derived stem cells, to potent embryonal carcinoma (EC) cells, or early cleavage embryos with viral expression vectors containing the recombinant nucleic acid molecule. (See for example, Palmiter et al. (1986) Ann.Rev. Genet. 20:465-499 and Capecchi (1989) Science 244:1288-1292.) Retroviral infection can also be used to introduce transgene into an animal, including swine. The developing embryo can be cultured in vitro to the blastocyst stage. During this time, the blastomeres can be targets for retroviral infection (Jaenich (1976) PNAS 73:1260-1264). Efficient infection of the blastomeres is obtained by enzymatic treatment to remove the zona pellucida (Hogan et al. (1986) in Manipulating the Mouse Embryo, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N. Y.). The viral vector system used to introduce the transgene is typically a replication-defective retrovirus carrying the transgene (Jahner et al. (1985) PNAS 82:6927-6931; Van der Putten et al. (1985) PNAS 82:6148-6152). Transfection can be obtained by culturing the blastomeres on a monolayer of virus-producing cells (Van der Putten, supra; Stewart et al. (1987) EMBO J. 6:383-388). Alternatively, infection can be performed at a later stage. Virus or virus-producing cells can be injected into the blastocoele (Jahner et al. (1982) Nature 298:623.628). Most of the founders will be mosaic for the transgene since incorporation typically occurs only in a subset of the cells which formed the transgenic swine. Further, the founder may contain various retroviral insertions of the transgene at different positions in the genome which generally will segregate in the offspring. In addition, it is also possible to introduce transgenes into the germ line, albeit with low efficiency, by intrauterine retroviral infection of the mid-gestation embryo (Jahner et al. (1982) supra). A third approach, which may be useful in the construction of tansgenic animals, would target transgene introduction into an embryonal stem cell (ES). ES cells are obtained from pre-implantation embryos cultured in vitro and fused with embryos (Evans et al. (1981) Nature 292:154-156; Bradley et al. (1984) Nature 309:255-258; Gossler et al. (1986) PNAS 83:9065-9069; and Robertson et al. (1986) Nature 322:445-448). Transgenes might be efficiently introduced into the ES cells by DNA transfection or by retrovirus-mediated transduction. Such transformed ES cells could thereafter be combined with blastocysts from the same species. The ES cells could be used thereafter to colonize the embryo and contribute to the germ line of the resulting chimeric animal. For review, see Jaenisch (1988) Science 240:1468-1474. Introduction of the recombinant gene at the fertilized oocyte stage ensures that the gene sequence will be present in all of the germ cells and somatic cells of the transgenic "founder" animal. As used herein, founder (abbreviated "F") means the animal into which the recombinant gene is introduced at the one cell embryo stage. The presence of the recombinant gene sequence in the germ cells of the transgenic founder animal in turn means that approximately half of the founder animal's descendants will carry the activated recombinant gene sequence in all of their germ cells and somatic cells. Introduction of the recombinant gene sequence at a later embryonic stage might result in the gene's absence from some somatic cells of the founder animal, but the descendants of such an animal that inherit the gene will carry the activated recombinant gene in all of their germ cells and somatic cells. MICROINJECTION OF SWINE OOCYTES In preferred embodiments the transgenic animals of the present invention, including but not limited to swine are produced by: i) microinjecting a recombinant nucleic acid molecule encoding a polypeptide into a fertilized egg to produce a genetically altered egg; ii) implanting the genetically altered egg into a host female animal of the same species; iii) maintaining the host female for a time period equal to a substantial portion of the gestation period of said animal fetus, iv) harvesting a transgenic animal having at least one cell that has developed from the genetically altered mammalian egg, which expresses a gene which encodes a polypeptide In general, the use of microinjection protocols in transgenic animal production is typically divided into four main phases: (a) preparation of the animals; (b) recovery and maintenance in vitro of one or two-celled embryos; (c) microinjection of the embryos and (d) reimplantation of embryos into recipient females. The methods used for producing transgenic livestock, particularly swine, do not differ in principle from those used to produce transgenic mice. Compare, for example, Gordon et al. (1983) Methods in Enzymology 101 :411, and Gordon et al. (1980) PNAS 77:7380 concerning, generally, transgenic mice with Hammer et al. (1985) Nature 315:680, Hammer et al. (1986) J Anim Sci 63:269-278, Wall et al. (1985) Biol Reprod. 32:645-651, Pursel et al. (1989) Science 244:1281-1288, Vize et al. (1988) J Cell Science 90:295-300, Muller et al. (1992) Gene 121 :263-270, and Velander et al (1992) PNAS 89:12003-12007, each of which teach techniques for generating transgenic swine. See also, PCT Publication WO 90/03432, and PCT Publication WO 92/22646 and references cited therein One step of the preparatory phase comprises synchronizing the estrus cycle of at least the donor females, and inducing superovulation in the donor females prior to mating. Superovulation typically involves administering drugs at an appropriate stage of the estrus cycle to stimulate follicular development, followed by treatment with drugs to synchronize estrus and initiate ovulation. As described in the example below, a pregnant female animal's serum is typically used to mimic the follicle-stimulating hormone (FSH) in combination with human chorionic gonadotropin (hCG) to mimic luteinizing hormone (LH). The efficient induction of superovulation depends, as is well known, on several variables including the age and weight of the females, and the dose and timing of the gonadotropin administration. See for example, Wall et al. (1985) Biol. Reprod. 32:645 describing superovulation of pigs. Superovulation increases the likelihood that a large number of healthy embryos will be available after mating, and further allows the practitioner to control the timing of experiments After mating, one or two-cell fertilized eggs from the superovulated females are harvested for microinjection. A variety of protocols useful in collecting eggs from animals are known. For example, in one approach, oviducts of fertilized superovulated females can be surgically removed and isolated in a buffer solution/culture medium, and fertilized eggs expressed from the isolated oviductal tissues. See, Gordon et al. (1980) PNAS 77:7380; and Gordon et al. (1983) Methods in Enzymology 101:411. Alternatively, the oviducts can be cannulated and the fertilized eggs can be surgically collected from anesthetized animals by flushing with buffer solution/culture medium, thereby eliminating the need to sacrifice the animal. See Hammer et al. (1985) Nature 315:600. The timing of the embryo harvest after mating of the superovulated females can depend on the length of the fertilization process and the time required for adequate enlargement of the pronuclei. This temporal waiting period can range from, for example, up to 48 hours for larger animal species. Fertilized eggs appropriate for microinjection, such as one-cell ova containing pronuclei, or two-cell embryos, can be readily identified under a dissecting microscope The " equipment and reagents needed for micro inj ection of the isolated embryos from larger animals are similar to that used for the mouse. See, for example, Gordon et al. (1983) Methods in Enzymology 101 :411; and Gordon et al. (1980) PNAS 77:7380, describing equipment and reagents for microinjecting embryos. Briefly, fertilized eggs are positioned with an egg holder (fabricated from 1 mm glass tubing), which is attached to a micro-manipulator, which is in turn coordinated with a dissecting microscope optionally fitted with differential interference contrast optics. Where visualization of pronuclei is difficult because of optically dense cytoplasmic material, such as is generally the case with swine embryos, centrifugation of the embryos can be carried out without compromising embryo viability. Wall et al. (1985) Biol. Reprod. 32:645. Centrifugation will usually be necessary in this method. A recombinant nucleic acid molecule of the present invention is provided, typically in linearized form, by linearizing the recombinant nucleic acid molecule with at least 1 restriction endonuclease, with an end goal being removal of any prokaryotic sequences as well as any unnecessary flanking sequences. In addition, the recombinant nucleic acid molecule containing the tissue specific promoter and the human class I gene may be isolated from the vector sequences using 1 or more restriction endonucleases. Techniques for manipulating and linearizing recombinant nucleic acid molecules are well known and include the techniques described in Molecular Cloning: A Laboratory Manual, Second Edition. Maniatis et al. eds., Cold Spring Harbor, N.Y. (1989). The linearized recombinant nucleic acid molecule may be microinjected into an egg to produce a genetically altered mammalian egg using well known techniques. Typically, the linearized nucleic acid molecule is microinjected directly into the pronuclei of the fertilized eggs as has been described by Gordon et al. (1980) PNAS 77:7380-7384. This leads to the stable chromosomal integration of the recombinant nucleic acid molecule in a significant population of the surviving embryos. See for example, Brinster et al. (1985) PNAS 82:4438-4442 and Hammer et al. (1985) Nature 315:600-603. The microneedles used for injection, like the egg holder, can also be pulled from glass tubing. The tip of a microneedle is allowed to fill with plasmid suspension by capillary action. By microscopic visualization, the microneedle is then inserted into the pronucleus of a cell held by the egg holder, and plasmid suspension injected into the pronucleus. If injection is successful, the pronucleus will generally swell noticeably. The microneedle is then withdrawn, and cells which survive the microinjection (e.g. those which do not lyse) are subsequently used for implantation in a host female The genetically altered mammalian embryo is then transferred to the oviduct or uterine horns of the recipient. Microinjected embryos are collected in the implantation pipette, the pipette inserted into the surgically exposed oviduct of a recipient female, and the microinjected eggs expelled into the oviduct. After withdrawal of the implantation pipette, any surgical incision can be closed, and the embryos allowed to continue gestation in the foster mother. See, for example, Gordon et al. (1983) Methods in Enzymology 101:411; Gordon et al. (1980) PNAS 77:7390; Hammer et al. (1985) Nature 315:600; and Wall et al. (1985) Biol. Reprod. 32:645 The host female mammals containing the implanted genetically altered mammalian eggs are maintained for a sufficient time period to give birth to a transgenic mammal having at least 1 cell, e.g. a bone marrow cell, e.g. a hematopoietic cell, which expresses the recombinant nucleic acid molecule of the present invention that has developed from the genetically altered mammalian egg At two-four weeks of age (post-natal), tissue samples are taken from the transgenic offspring and digested with Proteinase K. DNA from the samples is phenol-chloroform extracted, then digested with various restriction enzymes. The DNA digests are electrophoresed on a Tris-borate gel, blotted on nitrocellulose, and hybridized with a probe consisting of the at least a portion of the coding region of the recombinant cDNA of interest which had been labeled by extension of random hexamers. Under conditions of high stringency, this probe should not hybridize with the endogenous (non-transgene) genes, but should produce a hybridization signal in animals expressing the transgene, allowing for the identification of transgenic pigs PRODUCTION OF TRANSGENIC ANIMALS BY CLONING Transgenic animals for use in the present invention may also be made by other methods, for example, by cloning. Cloning by nuclear transfer to enucleated cells is described in US Patent No. 6,147,276, and in numerous publications, including Campbell et al, 1996, Nature 380 64-66; Wilmut et al, 1997, Nature 385 810-813; Schneike et al, 1997, Science 278 2130-2133; Ashworth et al., 1998, Nature 394 329; Sheils et al, 1999, Nature 399 316-317; and Evans et al, 1999, Nature Genetics 23 90-93. For example, in order to clone an animal, the following technique may be used. Unfertilised eggs are flushed out of a female animal, which may be induced to produce a larger than normal number of eggs. A sample of tissue is taken from a suitable part of a donor animal (for example, adult tissue such as udder tissue or embryonic tissue) and cultured in vitro. Cultured cells are then starved to send them into a resting or quiescent state by, for example, serum starvation) The donor cell is then fused or injected into the recipient cell. For example, a cell from the culture is placed beside the egg and an electric current used to fuse the couplet. The reconstructed embryo is put into culture and allowed to grow for a length of time (for example, seven days). The recipient cell is activated before, during or after nuclear transfer. Embryos which grow successfully are taken and transferred to a recipient animal which is at the same stage of the oestrus cycle as the egg. The recipient animal becomes pregnant and produces a cloned animal after a suitable gestation period. Direct microinjection of donor cell nuclei may also be used (the so-called "Honolulu technique"). Direct microinjection of a nucleus from an adult cell into an oocyte from which the nucleus has already been removed has been used to clone mice. The eggs are then prevented from dividing and forming multicelled blastocysts for periods of time (for example, from one to six hours) and subsequently allowed to divide. Cloning using nuclear transfer from established cell lines is described in Nature 380, 64-66, and also in International Patent Application Numbers PCT/GB96/02099, and PCT/GB96/02098. Transgenic lambs producing recombinant blood clotting factor IX have also been produced. Delayed activation of donor cells is described in UK Patent Numbers GB 2318792 and GB 2340493. Knock-out Technology In addition to the addition of exogenous genes to RBCs, a further embodiment of the present invention includes the potential for deletion of genes from RBCs, wherein the deletion provides a therapeutic advantage. For example, it may be advantageous to delete one or more cell surface blood group antigens or epitopes using gene knock out techniques in order to avoid or lessen a host immune response to administered RBCs. i. Standard Knock Out Animals Knock out animals are produced by the method of creating gene deletions with homologous recombination. This technique is based on the development of embryonic stem (ES) cells that are derived from embryos, are maintained in culture and have the capacity to participate in the development of every tissue in the animals when introduced into a host blastocyst. A knock out animal is produced by directing homologous recombination to a specific target gene in the ES cells, thereby producing a null allele of the gene, GCK/IRS1, IRS1/LNSR, MC4R (Huszar et al., 1997, Cell, 88:131) and BRS3 (Ohki-Hamazaki et al., 1997, Nature, 390:165) ii. Tissue Specific Knock Out The method of targeted homologous recombination has been improved by the development of a system for site-specific recombination based on the bacteriophage PI site specific recombinase Cre. The Cre-loxP site-specific DNA recombinase from bacteriophage PI is used in transgenic mouse assays in order to create gene knockouts restricted to defined tissues or developmental stages. Regionally restricted genetic deletion, as opposed to global gene knockout, has the advantage that a phenotype can be attributed to a particular cell/tissue (Marth, 1996, Clin. Invest. 97: 1999). In the Cre- loxP system one transgenic mouse strain is engineered such that loxP sites flank one or more exons of the gene of interest. Homozygotes for this so called 'floxed gene' are crossed with a second transgenic mouse that expresses the Cre gene under control of a cell/tissue type transcriptional promoter. Cre protein then excises DNA between loxP recognition sequences and effectively removes target gene function (Sauer, 1998, Methods, 14:381). There are now many in vivo examples of this method, including the inducible inactivation of mammary tissue specific genes (Wagner et al., 1997, Nucleic Acids Res. , 25:4323) iii. Bac Rescue of Knock Out Phenotype In order to verify that a particular genetic polymorphism/mutation is responsible for altered protein function in vivo one can "rescue" the altered protein function by introducing a wild-type copy of the gene in question. In vivo complementation with bacterial artificial chromosome (BAC) clones expressed in transgenic mice can be used for these purposes. This method has been used for the identification of the mouse circadian Clock gene (Antoch et al, 1997, Cell 89: 655). NUCLEIC ACID A nucleic acid of use in the invention (whether as, or to encode, an agent for delivery, an expression vector, etc) may comprise a viral or non- viral DNA or RNA vector, where non- viral vectors include, but are not limited to, plasmids, linear nucleic acid molecules, artificial chromosomes, condensed particles and episomal vectors. Expression of heterologous genes has been observed after injection of plasmid DNA into muscle (Wolff J. A. et al, 1990, Science, 247: 1465-1468; Carson D.A. et al, US Patent No. 5,580,859), thyroid (Sykes et al, 1994, Human Gene Ther., 5: 837-844), melanoma (Vile et al, 1993, Cancer Res., 53: 962-967), skin (Hengge et al, 1995, Nature Genet., 10: 161-166), liver (Hickman et al., 1994, Human Gene Therapy, 5: 1477-1483) and after exposure of airway epithelium (Meyer et al, 1995, Gene Therapy, 2: 450-460). As used herein, the term "nucleic acid" is defined to encompass DNA and RNA or both synthetic and natural origin which DNA or RNA may contain modified or unmodified deoxy- or dideoxy- nucleotides or ribonucleotides or analogues thereof. The nucleic acid may exist as single- or double-stranded DNA or RNA, an RNA/DNA heteroduplex or an RNA/DNA copolymer, wherein the term "copolymer" refers to a single nucleic acid strand that comprises both ribonucleotides and deoxyribonucleotides . The term "synthetic", as used herein, is defined as that which is produced by in vitro chemical or enzymatic synthesis. Therapeutic nucleic acid sequences useful according to the methods of the invention include those encoding receptors, enzymes, ligands, regulatory factors, and structural proteins. Therapeutic nucleic acid sequences also " include sequences encoding nuclear proteins, cytoplasmic proteins, mitochondrial proteins, secreted proteins, plasmalemma-associated proteins, serum proteins, viral antigens, bacterial antigens, protozoal antigens and parasitic antigens. Therapeutic nucleic acid sequences useful according to the invention also include sequences encoding proteins, lipoproteins, glycoproteins, phosphoproteins and nucleic acids (e.g., RNAs such as ribozymes or antisense nucleic acids). Ribozymes of the hammerhead class are the smallest known, and lend themselves both to in vitro synthesis and delivery to cells (summarised by Sullivan, 1994, J Invest. Dermatol, 103: 85S-98S; Usman et al, 1996, Curr. Opin. Struct. Biol, 6: 527-533). Proteins or polypeptides which can be expressed by nucleic acid molecules delivered according to the present invention include neurotransmitters, enzymes, immunoglobulins, antibodies, toxins, apolipoproteins, receptors, drugs, oncogenes, tumour antigens, tumour suppressers, structural proteins, viral antigens, parasitic antigens and bacterial antigens. The compounds which can be incorporated are only limited by the availability of the nucleic acid sequence encoding a given protein or polypeptide. One skilled in the art will readily recognise that as more proteins and polypeptides become identified, their corresponding genes can be cloned into the gene expression vector(s) of choice, administered to a tissue of a recipient patient or other vertebrate, and expressed in that tissue. KITS The invention also encompasses a number of kits. Some of the kits comprise partially or fully treated red blood cells. Other kits provide a red blood cell, preferably a sensitised red blood cell, an agent-MTS conjugate to be loaded and packaging materials therefor (optionally together with instructions for carrying out the methods of the invention). A kit designed for the easy delivery of an agent to a recipient vertebrate, whether in a research of clinical setting, is encompassed by the present invention. A kit takes one of several forms, as follows: A kit for the delivery of an agent to a subject vertebrate comprises preferably sensitised red blood cells and the agent and optionally instructions for loading the agent-MTS conjugate. Alternatively, the red blood cells are supplied loaded with the agent-MTS conjugate for convenience of use by the purchaser. In the latter case, the cells may be supplied in sensitised form, ready for rapid use or pre-sensitised and loaded but needing a final sensitisation step. The cells of the kit are typically species-specific to the vertebrate of interest, such as a primate, including a human, canine, rodent, mouse, rat, rabbit, sheep, goat, horse, cow, and pig or other, as desired; in other words, the cells are of like species with the intended recipient. The cells of the kit are, additionally, specific to the blood type of the intended recipient organism, as needed. Optionally, the kit comprises one or more buffers for cell sensitisation, pre-sensitisation, washing, re-suspension, dilution and/or administration to a vertebrate. Appropriate buffers are selected from the group that includes low ionic strength saline, physiological buffers such as PBS or Ringer's solution, cell culture medium and blood plasma or lymphatic fluid. The kit additionally comprises packaging materials (such as tubes, vials, bottles, or sealed bags or pouches) for each individual component and an outer packaging, such as a box, canister or cooler, which contains all of the components of the kit. The kit may be shipped refrigerated. Optionally, non-cellular components are supplied at room temperature or frozen, as needed to maintain their activity during storage and shipping. They may be in liquid or dry (i.e., powder) form. A second kit of the invention comprises an agent such as a biological effector molecule, instructions for performing the method of the present invention and, optionally a sensitising device and buffers therefor (e.g., saline or other physiological salt buffer, culture medium, plasma or lymphatic fluid). In addition, the kit contains appropriate packaging materials, as described above. The individual components may be supplied in liquid or dry (i.e., powder) form, and may be at room temperature, refrigerated or frozen as needed to maintain their activity during storage and shipping. Red blood cells for use with this kit may be obtained independently (for example, they may be harvested from the intended recipient vertebrate). A preferred aspect of the invention is a kit comprising a red blood cell which is loaded with an agent, and packaging materials therefor. Preferably, a kit as described above further comprises an apparatus for applying the sensitising procedure. Preferably a kit of the invention further comprises an immuno globulin or polyethylene glycol. Preferably the kit further comprises a liquid selected from a buffer, diluent or other excipient. More preferably the liquid is selected from a saline buffer, a physiological buffer and plasma. Another aspect of the invention is a physiological composition comprising a red blood cell delivery vector of the invention comprising an agent such as a biological effector molecule. The red blood cell is admixed with a pharmaceutically acceptable carrier or diluent, or a physiologically compatible buffer. As used herein, the term "physiologically compatible buffer" or "physiological buffer" is defined as a liquid composition which, when placed in contact with living cells, permits the cells to remain alive over a period of minutes, hours or days. As such, a physiological buffer is substantially isotonic with the cell, such that cell volume does not change more than 20% due to differences in internal and external ionic strength. Non-limiting examples of physiologically compatible buffers or physiological buffers include dilute saline, which may be buffered (e.g., Hanks' buffered saline or phosphate buffered saline), or other physiological salts (e.g., Ringer's solution), dilute glucose, sucrose or other sugar, dilute glycerol with- or without salts or sugars, cell culture media as are known in the art, serum and plasma. Preferably, the red blood cell of the physiological composition is a human cell. IMMUNISATION The present invention also includes the use of red blood cells comprising an agent in a method of immunisation of an animal. According to this embodiment, an agent comprising an antigen is loaded into a red blood cell as described above. The loaded red blood cell may then be sensitised to render it more susceptible to disruption by exposure to a stimulus than an unsensitised red blood cell. A preferred method of sensitisation is electrosensitisation, as described above. The loaded red blood cell is then introduced into an animal, optionally together with an adjuvant. The animal, or a portion of the animal, is then exposed to an appropriate source of energy to disrupt the red blood cells. Preferably, the source of energy is ultrasound energy, as described in detail above. A single administration/release may be employed, or repeated administrations followed by repeated releases may be employed. The advantage of the immunisation regime according to this aspect of the invention is that no immune response is induced in the animal until the agent (antigen) is released. The embodiment comprising repeated release is especially suitable for priming and boosting regimes to ensure a high immune response. It will be appreciated that the antigens involved need not comprise membrane translocation sequences; indeed, any agent capable of being loaded into a red blood cell is suitable for use as an antigen according to this aspect of the invention. EXAMPLES RBC preparation, electrosensitisation and ultrasound treatment protocols are as described in Example 1 (second procedure) of PCT/GBOO/03056. Unless indicated otherwise, the following Examples utilise these protocols. Where dialysis loading is indicated, the protocol described in Example 1 (second procedure) of PCT/GBOO/03056 is utilised. Autoloading protocols involve incubation of the agent to be loaded with the red blood cells, and are described in detail below. Bax buffer and Bax modified buffer are described in Bax et al (1999), Clinical Science 96, 171-178 and have the following compositions: Bax-modified buffer (mBAX: PH 7.4; 2.68mM KC1, 1.47M KH 2 PO 4 , 136mM NaCl, 8.1mMNa 2 HPO 4, 5mM glucose, 5mM adenine, 5mM MgCl 2 . Check and adjust pH with IM NaOH); Bax buffer (BAX: PH 7.4; , 2.68mM KC1, 1.47M KH 2 PO 4 , 136mM NaCl, 8-lmM Na 2 HPO 4ι 5mM glucose, 5mM adenosine, 5mM MgCl 2 . Check and adjust pH withlM NaOH). Example 1. Circulating Phantom A circulating phantom system assays the ultrasound sensitivity of cells in a context which mimics a physiological environment. The release of a payload when treated with ultrasound may therefore be monitored in a system which imitates a circulatory system. Cells are spiked into whole blood (approximately 2%>) and circulated in a physiological buffer, at a flow rate which is similar to the central venous flow rate (approximately 15ml/min) of a relevant organism, and at a temperature which is close to or identical to the body temperature of the animal in question. The particular circulating phantom system as used in the Examples described below comprises a bath maintained at a suitable temperature, circulating means in which red blood cells may be circulated, and a ultrasound source. The ultrasound source comprises an ultrasound head which in use is placed adjacent to a wall of the bath. A portion of the circulating means is placed inside the bath, and the ultrasound head transmits ultrasound energy across the wall of the bath and the wall of the tubing to the red blood cells maintained in there. The bath is maintained at 37 degrees C (or any other suitable temperature) by an immersion heater and a thermostat. The bath contains water or a buffer such as PBS, although any liquid which has adequate temperature buffering capacity may also be used. The walls of the bath are constructed of for example plastic sheeting, although at least a portion of one or more walls should be constructed of a material which substantially allows the passage of ultrasound preferably without significant attenuation. The bath may therefore comprise a window in one of the walls allowing passage of ultrasound. A suitable ultrasound transmitting material includes builders plastic obtainable from a hardware store. The circulating means enables red blood cells to be moved across the ultrasound field and enables exposure of cells to ultrasound. The circulating means preferably comprises tubing, for example ordinary laboratory plastic tubing. The tubing comprises at least a portion which is capable of transmitting ultrasound, and may be transparent or translucent to visible light. The tubing as used in the Examples comprises a section of ultrasound transmitting material (C-Flex™ tubing, made by Cole Parmer, UK) linked to a section of laboratory tubing. This is inserted into a peristaltic pump to drive the cells around the tubing. The bottom of the peristaltic tubing which acts as the target vessel is situated at 1.3cm above the head of the probe. In a preferred embodiment, the bath is cylindrical and the bottom of the bath consists of a light polyethylene sheet through which ultrasound is delivered. The blood is circulated through C-Flex™ tubing (internal diameter 4mm) which passes through the thermostated buffer and the target area of the C-flex tubing is positioned at a distance of 1.3 cm from the ultrasound-emitting head. Blood is circulated through the system at a rate of 14.5ml/min. During exposure to ultrasound (for example, 5W/cm 2 at 1MHz for indicated times), samples are harvested from the system and supernatants are harvested by centrifugation. A protocol for initial equipment set-up is as follows: 1. Unscrew the bolts from the bottom and separate the top and bottom plates, and grease the rubber seals on the inside of each plate with Vaseline; 2. Cut a piece of builder's polythene to size and place tightly over the bottom plate. Replace the top plate and line up the screw holes. Replace screws to hold the polythene in place, making sure that the polythene remains as taut as possible; 3. Place some water into the unit to make sure that it is water tight. If not, unscrew the device and try again. 4. Place tubing onto water inlet and outlet on the CP and connect to the pumps. 5. Ensure that the ultrasound probe head is level, and securely held by a clamp. Place ultrasound gel on the probe head. 6. Clamp the CP chamber to a stand and place over the probe head- keep probe head as central as possible, making sure there are no air bubbles between the probe head and the surface of the polythene, and making sure that the surface is level. 7. Secure in place. 8. Place the two free ends of the inlet and outlet tubes into a water bath, set at 40°C. This will allow temperature in the CP to stay at approximately 37°C. Verified with a thermometer. 9. Switch the pumps on and allow the water to circulate- check the temperature and that the water level remains constant, at approximately 1 cm below the inlet pipe. Sample Set-up is as follows: 1. Clamp on sample bar containing peristaltic tubing. Set the bottom of the tubing to 1.3cm over the centre of the probe head. 2. Flush out the tubing with 20mL PBS solution. 3. Once the tubing is clean, force out the PBS solution with air. 4. Immerse one end of the peristaltic tubing into 3mL of freshly collected, washed whole blood (sample coming from same animal as loaded cells). 5. Slowly, pull the sample through the peristaltic tubing using the peristaltic pump controlling the flow, ensuring no air is allowed into the system. 6. Adjust the flow through the tubing to 15mL/min. The peristaltic pump is marked with this flow. 7. Allow 15 minutes for the system to equilibrate to the temperature of the CP chamber. 8. Remove 0.6mL of blood from the system. This will act as a control. 9. To the circulating blood, add 0.6mL of loaded cells prepared according to the respective loading method, at 7x10 8 c/mL. This corresponds to a 2% spike. 10. The system is now ready to be used for application of ultrasound. Example 2. Loading Membrane Translocation Sequence Peptides Into Electrosensitised Erythrocytes The objective of experiments described in Examples 2 to 8 is to demonstrate that the peptides penetratin, HIV-TAT and VP22 may be incorporated into electrosensitised erythrocytes. In this study uptake by erythrocytes from a number of sources including human, pig, rabbit and mouse is examined. The penetratin payload comprises a FITC-Penetratin conjugate, having the following sequence: Fluorescein-RQIKIWFQNRRMKWKKC (custom made by Altabioscience, Southampton, UK). The HIV-TAT fragment has the following sequence: Fluorescein-GRKKRRQRRRPPQC-amide (2181.5 Da). VP22 as used here has the following sequence: NAATATRGRSAASRPTERPRAPARSASRPRRPVEC-amide. VP22 is obtained from Alta Biosciences, Edgbaston, Birmingham. Whole blood from rabbit is collected in heparinised containers and cells are washed and sensitised. The cell concentration is adjusted to 1.5 x 10 9 and fiuorescein- labelled penetratin, HIV-TAT fragment and VP22 are added at the indicated concentrations (in PBS) and mixtures are incubated for 30 min at 37°C. The mixtures are then centrifuged at 700 g for 5 minutes and the cells are resuspended with PBS-Mg-Glucose (rabbit and mouse) or mBAX (human and pig), and subsequently washed twice. Uptake of peptide is monitored by analysis on flow cytometry where this uptake of the fluorescent peptide is indicated by a shift to the right on the flow cytometry profiles. Results 2 The results are shown in Figures 1 A (HIV-TAT), IB (penetratin) and 1C (VP- 22) where increasing concentrations of peptide results in increasing shifts to the right on the flow cytometry profiles. In each case the progressive shift the right is indicative of peptide uptake by the sensitised carrier vehicle. In all cases, the peptide-loaded cells are shown to be preferentially sensitive to low intensity ultrasound (100%) lysis when treated with ultrasound at 3 W/cm 2 and at 1MHz using the TMM system; TMM is a tissue mimicking material which attenuates ultrasound in the same manner as a soft tissue. The TMM chosen for this work is described in Madsen et al. (1998, Ultrasound Med. & Biol, 24, 535-542) and following preparation, care is taken to ensure that the material has a density of 1.03g/ml). These results demonstrate that peptides comprising membrane translocation sequences may be auto-loaded into electrosensitised erythrocytes and uptake of peptide is dependant on peptide concentration. Example 3. Stability of Peptide in a Red Blood Cell Vehicle The objective of these experiments is to demonstrate that, once loaded, the membrane translocation sequence peptides are stable in the vehicle even following incorporation into whole blood. Erythrocytes loaded with HIV-TAT fragment from human, rabbit, pig and mouse (as described in the preceding Example) are spiked (1%) into whole blood of the corresponding species. Stability at 4°C is assessed for up to 24 hours using flow cytometry, where cells counts of the loaded cell population are analysed against time. Results 3 The results are shown in Figures 2 A-D where loaded human (Figure 2A), rabbit (Figure 2B), pig (Figure 2C) and mouse (Figure 2D) cells are spiked into whole blood. In all cases, cell counts of the loaded cell population remain constant throughout the course of the experiment. The results demonstrate that once loaded, the membrane translocation peptides are stable in the vehicle even following incorporation into whole blood. Example 4. Ultrasound-Mediated Release and Bioactivity of a Released Peptide The objective of these experiments is to demonstrate that the relevant peptide can be released from the vehicle using ultrasound and to further demonstrate that the peptide retains its function in terms of its ability to enter target cells. White blood cells are prepared by buoyant density centrifugation Histopaque 1077 (Sigma). Cells are harvested and washed three times in mBax and stored on ice until use. 350 μl of loaded RBC with 0.1 mg/ml penetratin HIV-TAT fragment are treated on TMM ultrasound 1 MHz Probe, 3 Watts/cm 2 . Samples of the lysates are pooled and centrifuged down to remove debris. WBC populations are incubated together with i) buffer, ii) penetratin and iii) lysates derived from ultrasound treated, penetratin loaded vehicle. Samples are then analysed using flow cytometry and lymphocyte populations are resolved. Uptake of penetratin by this population is indicated by a shift to the right on flow cytometry profiles. Results 4 The results are shown in Figure 3. These illustrate that free penetratin is taken up by the lymphocyte population and the shift of the profile to the right serves as a positive control. In addition, the results demonstrate that penetratin, released from the erythrocyte vehicle, is also taken up by the target lymphcyte population. These results demonstrate that ultrasound facilitates the release of bioactive penetratin from a loaded, sensitised vehicle. Example 5. Uptake of a Penetratin-Phosphorothioate Backbone Oligonucleode Conjugate By Electrosensitised Erythrocytes The objective of these experiments is to demonstrate that the an agent-MTS conjugate, namely, an oligonucleotide conjugated to penetratin, may be incorporated into electrosensitised erythrocytes. In this study uptake by erythrocytes from human is examined. Whole blood from human is collected in heparinised containers and cells are washed and sensitised as described in Example 1 (second procedure) of PCT/GBOO/03056.. The cell concentration is adjusted to 7.0 x 10 8 and a FITC-penetratin- phosphorothioate backbone oligonucleode conjugate (Alta Biosciences, Edgbaston, Birmingham) is added at 0.05 mg/ml, O.lmg/ml, 0.15 mg/ml and 0.2 mg/ml (in PBS) and mixtures are incubated for 30 min at 37°C. The mixtures are then centrifuged at 700 g for 5 minutes and the cells are resuspended with mBAX, and subsequently washed twice. Uptake of peptide is monitored by analysis on flow cytometry where this uptake of the fluorescent peptide is indicated by a shift to the right on the flow cytometry profiles. Results 5 The results are shown in Figure 5 where increasing concentrations of peptide results in increasing shifts to the right on the flow cytometry profiles. In each case the progressive shift the right is indicative of peptide uptake by the sensitised carrier vehicle. These results demonstrate that membrane translocation sequence peptide- conjugates may be auto-loaded into electrosensitised erythrocytes and uptake is dependant on concentration. Examples 6, 7 and 8 describe loading of FITC-labelled HIV-TAT fragment into electrosensitised erythrocytes by dialysis, ultrasound mediated release of payload in whole circulating blood in vitro, ultrasound mediated release in an in vivo model, and demonstration that circulating cells remain sensitised. Example 6. Loading of FITC-Labelled HIV-TAT Fragment into Electrosensitised Erythrocytes by Hypotonic Dialysis (Stability) The objective of these experiments is to demonstrate that the peptide HIV-TAT fragment may be incorporated into electrosensitised erythrocytes, using dialysis loading. Whole blood from pig is collected in heparinised containers and cells are washed and sensitised as described in Example 1 (second procedure) of PCT/GBOO/03056. The cell concentration is adjusted to 7 x 10 8 . Cells are washed once in PBS at 700g for 5 min, and once in buffer (isoosmotic PBS: pH7.4 K 2 H/KH 2 phosphate buffer, with 150mM NaCl; 8.76g/L; check and adjust pH with IM NaOH) at 700g for 5 mins. The cells are retained as a packed cell volume and fluorescein-labelled HIV-TAT fragment (Alta Biosciences, Edgbaston, Birmingham) is added to the packed cell volume at the indicated concentrations (expressed as mg/ml peptide to 7 x 10 cells/ml) ' and mixtures placed in dialysis tubing (1 OOODa MW tubing, Spectro-Por, Spectrum Inc.,) for 60 min at 37°C. Cells are then dialysed against buffer 2 (dialysis buffer: pH7.4 K 2 H/KH phosphate buffer; check and adjust pH with IM NaOH) for one hour at 4°C. Membranes are then placed into mBAX at 37°C, and dialysed for one hour. Cells are harvested from the dialysis membranes and washed three times in mBAX buffer at 170g for 15 minutes at room temperature. Cells are re-suspended at 7 x 10 8 in mBAX and stored at 4°C overnight. The next day, cells are washed and sensitised as described in Example 1 (second procedure) of PCT/GBOO/03056, and the cell concentration is adjusted to 7 x 10 8 cells/ml. Uptake of peptide is monitored by analysis on flow cytometry where this uptake of the fluorescent peptide is indicated by a shift to the right on the flow cytometry profiles. In order to assess stability of loaded red blood cells, erythrocytes loaded with HIV-TAT fragment are spiked (1%) into whole blood. Stability at 37°C and 4°C is assessed for up to 24 hours using flow cytometry, where cells counts of the loaded cell population are analysed against time. Results 6 The loading results are shown in Figure 5 where increasing concentrations of peptide result in increasing shifts to the right on the flow cytometry profiles. In each case the progressive shift the right is indicative of peptide uptake by the sensitised carrier vehicle. In all cases, the peptide-loaded cells are shown to be preferentially sensitive to low intensity ultrasound (100%o lysis when treated with ultrasound at 3 W/cm " and at 1MHz using a Tissue Mimicking Medium system as described briefly here. In a TMM system, the target is placed at a distance of 1.3cm from the emitting surface of the ultrasound head and the intervening space is filled with a tissue mimicking material (TMM) which attenuates ultrasound in the same manner as a soft tissue. The TMM chosen for this work is described in Madsen et al. (1998, Ultrasound Med. & Biol, 24, 535-542) and following preparation, care is taken to ensure that the material has a density of 1.03 g/ml. These results demonstrate that HIV-TAT fragment may be auto-loaded into electrosensitised erythrocytes more effectively by dialysis loading, and uptake of peptide is dependant on peptide concentration. Results relating to RBC stability are shown in Figures 6A and 6B where loaded pig cells are spiked into whole porcine blood. In both cases, cell counts of the loaded cell population remain constant throughout the course of the experiment. The results demonstrate that once dialysis loaded, the HIV-TAT fragment is stable in the vehicle even following incorporation into whole blood. Example 7. Ultrasound Mediated Release of Payload in Whole Circulating Blood in vitro The objective of this experiment is to demonstrate that the relevant peptide can be released by ultrasound from the vehicle in an in vitro circulating model, at 37 degrees C, 1.3cm from the ultrasound probe, spiked into whole blood. From this, ultrasound parameters may be established for further use in an in vivo system. Erythrocytes, electrosensitised and dialysis loaded with HIV-TAT fragment are spiked (2.5%) into whole blood of the same animal. A 3ml sample is then applied to the circulating phantom model at 4.5 - 6W/cm 2 (pulsed wave; 35%) for 15 min, and lOOμl samples collected for the circulating system every 5 min. Any ultrasound mediated decrease in loaded erythrocytes is demonstrated by loss of cells on the flow cytometer. Haemoglobin levels in the supernatants of the collected samples are assessed at Abs 54 o nm on the spectrophotometer. As controls, erythrocytes (non electrosensitised) and dialysis loaded with HIV- TAT fragment are spiked (2.5%) into whole blood of the same animal. A 3ml sample is then applied to the circulating phantom model at 5-8 W/cm 2 (pulsed wave; 35%) for 15 min, and lOOμl samples collected for the circulating system every 5 min. Haemoglobin levels in the supernatants of the collected samples are then assessed. Results 7 Figure 7A demonstrates that under the parameters used, an ultrasound intensity of 4.5 W/cm 2 confers negligible effects on the number of loaded cells in whole blood. At 5 W/cm 2 a decrease in the number of loaded cells occurs after 10 min, whereas at 5.5 and 6W/cm 2 , this time is reduced to 5 minutes. Figure 7B demonstrating haemoglobin release at the various ultrasound intensities, shows that release of this cell lysis marker mirrors the loss of labelled cells, showing that these cells are being targeted by ultrasound. These results show that in the in vitro circulating system, the loaded vehicle, spiked into whole blood is sensitive to ultrasound. Figure 7C illustrates that non electrosensitive, HIV-TAT fragment loaded pig erythrocytes display no changes in haemoglobin release when subjected to conditions of pulsed wave ultrasound at 5-7 W/cm 2 i.e., no ultrasound mediated lysis of non sensitised cells occurs. Combined with the information in Figure 7A, this establishes that a therapeutic window of between 5 to 7 W/cm 2 may be used in an in vivo model, to induce ultrasound mediated release of peptide payload in electrosensitised loaded cells Example 8. Ultrasound Mediated Release of Payload Release in vivo in Pig, and Demonstration that a Circulating Cell Remains Sensitised The objective of this experiment is to demonstrate that the relevant peptide is released by ultrasound from the vehicle in an in vivo model. Secondly, we investigate whether the loaded cells collected from the circulation still retain ultrasound sensitivity in an in vitro system. The presence of any in vivo repair processes to the loaded vehicle may be identified. Whole blood from pig is collected in heparinised containers. Cells are washed once in PBS at 700g for 5 min, and once in buffer 1 at 700g for 5 mins. The composition of buffer 1 (isoosmotic PBS) is as follows: pH7.4 K H/KH 2 phosphate buffer with 150mM NaCl (8.76g/l); check and adjust pH with IM NaOH. The cells are retained as a packed cell volume and 0.4 mg of fluorescein- labelled HIV TAT (Alta Biosciences, Edgbaston, Birmingham) is added for every 7 x 10 8 cells. The mixtures placed in dialysis tubing (1 OOODa MW tubing, Spectro-Por, Spectrum Inc.) for 60 min at 37°C. Cells are then dialysed against buffer 2 for one hour at 4°C. The composition of buffer 2 (dialysis buffer) is as follows: pH7.4 K 2 H/KH 2 phosphate buffer; check and adjust pH withlM NaOH. Membranes are then placed into mBAX at 37°C, and dialysed for one hour. Cells are harvested from the dialysis membranes and washed three times in mBAX buffer at 170g for 15 minutes at room temperature. Cells are resuspended at 7 x 10 8 in mBAX and stored at 4°C overnight. The next day, cells are washed as described in Example 1 (second procedure) of PCT/GBOO/03056. The cell concentration is adjusted to 7 x 10 8 cells/ml. The test system comprises two healthy, mature pigs of a crossbreed type (Large While x Landrace) of the male sex at least four weeks of age, each weighing 10kg. Venous puncture of the jugular vein of each animal enables 35mls of whole blood to be available for processing i.e., electrosensitisation and dialysis loading with fluorescently labelled HIV-TAT fragment. Anaesthesia is induced by injection of pentobarbitone at a dose rate of approx. 25mg/kg bodyweight (Sagatal (Merial)). The exterior ileal vein is catheterised and fitted with a 3 way tap, for sample adminstration and sampling. Preadministration samples are collected, prior to the test system receiving the processed packed cells, by slow intravenous injection (5 ml). In one of the subjects, after 60 min, ultrasound is applied to the jugular/carotid region of the neck at 6W/cm 2 (pulsed wave; 35%) (RICH-MAR CRM-1 machine fitted with a 1MHz head, for 3 x 10 min bursts, with a 1 minute rest between each 10 minute burst. The surface of this area is liberally covered with Alpha Lube gel (Ultrasonic Scanning gel, BCF Technology Ltd) before application of the head. Samples are collected at the time periods shown, and analysed using flow cytometry, where cells counts of the loaded cell population are assessed. Additional samples are collected, 10 minutes following cell administration to the animal, for application to the in vitro circulating model. Samples are collected 10 minutes following cell administration, from the circulating system, and assessed using flow cytometry, where cells counts of the loaded cell population are analysed Results 8 Figure 8A demonstrates that a clear increase in percentage loaded cells coincides with administration of loaded vehicle into the animal. In both subjects, cell number decreases quite significantly, between 5 and 10 minutes following adminstration. Spiking of a comparable volume of loaded cells into whole blood however would suggest that the 5 -minute sample may not have been an accurate representation, with insufficient dilution of the loaded cells. In the control animal, to which no ultrasound is applied, a gradual decline in labelled cell number is observed. In contrast, the effect of ultrasound on loaded cells in vivo is pronounced, and a dramatic decrease is shown between 2 and 5 minutes of ultrasound treatment at 6 W/cm 2 , pulsed wave; 35%. Figure 8B illustrates that samples collected 10 minutes following cell administration to the animal, for application to the in vitro circulating model, still show a decrease in loaded cell number with ultrasound treatment. This would suggest that in vivo repair processes during circulation are negligible, and the loaded vehicle still demonstrates ultrasound sensitivity. Example 9. Effect of Ultrasound on Non-Electrosensitised HIV-TAT Loaded Vehicle in vivo in Pig (Jugular Region) The objective of this experiment is to demonstrate that a loaded vehicle which has not been electrosensitised does not release its loaded components in vivo. Red blood cell collection, preparation, dialysis loading, the test system and ultrasound administration are as described above in the preceding Example, with the exception that electrosensitisation does not take place. Samples are collected at the time periods shown, and analysed using flow cytometry, where cells counts of the loaded cell population are assessed. Results 9 Figure 9 demonstrates that a clear increase in number of loaded cells coincides with administration of loaded vehicle into the animal. A gradual decline in labelled cell number is observed, prior to the administration of ultrasound, and during and after the 3 x 10 min bursts, under the parameters used, no effect on cell number could be observed. This would suggest that the as the cells are not resensitised, they are not receptive to ultrasound mediated lysis and subsequent release of payload, as would be predicted. Example 10. Effect of Ultrasound Targeting of the Jugular Region of a Pig Animal Model, on Kinetics of Payload Release in Vivo The objective of this experiment is to establish kinetics of payload release, when ultrasound is applied to the jugular region of the animal model. The test system comprises one animal, a description of which including blood collection, " loading conditions, anaesthesia and sample collection conditions has been provided in the above Examples, with the following modifications. After 30 minutes, ultrasound is applied to the jugular region of the neck at 6W/cm 2 (pulsed wave; 35%) (RICH-MAR CRM-1 machine fitted with a IMHz head, for a 8 x 1 min applications, each minute application followed by 4 minutes rest period. The surface of this area is liberally covered with Alpha Lube gel (Ultrasonic Scanning gel, BCF Technology Ltd) before application of the head. Samples are collected at the time periods shown, and analysed using flow cytometry, where cells counts of the loaded cell population are assessed. Results 10 Figure 10A (overall profile) demonstrates that following sample administration to the animal, the cells remain stable until the onset of ultrasound application. In the interim period of 2 and 3 minutes following ultrasound treatment, the number of loaded cells decreases completely. Figure 10B (detailed graph) shows that the 1 st 1 -minute ultrasound application does not have any direct effect on loaded cell number. The decrease in loaded cell number occurs directly after the 2 nd 1 -minute application, and this cumulative effect continues throughout the 3 rd application of ultrasound until no loaded cells remain. Example 11. Effect of Ultrasound Targeting of the Hepatic Artery Region of a Pig Animal Model on Subsequent Release of Payload Release in Vivo The objective of this experiment is to demonstrate that the relevant peptide can be released quickly from the vehicle, by ultrasound targeting of the hepatic artery region in an in vivo pig model. The test system comprises one animal description of which including blood collection, loading conditions, anaesthesia and sample collection conditions have been described in Example 3 above. After 90 min, ultrasound is applied to the hepatic artery/posterior vena cava/hepatic portal vein region of the neck at 6 W/cm 2 (pulsed wave; 35%>) (RICH- MAR CRM-1 machine fitted with a IMHz head, for a 4 min application. The surface of this area is liberally covered with Alpha Lube gel (Ultrasonic Scanning gel, BCF Technology Ltd) before application of the head. Samples are collected at the time periods shown, and analysed using flow cytometry, where cells counts of the loaded cell population are assessed. Results 11 Figure 11 demonstrates that a clear increase in loaded cell number as determined by flow cytometry (y axis) coincides with administration of loaded vehicle into the animal. The cells retain their sensitivity for 90 mins, where ultrasound is applied. Almost immediately, the loaded cells disappear, before the sample collected at 92 mins (i.e. 2 mins ultrasound treatment). Example 12. Effect of Ultrasound Targeting One of the Kidneys of a Pig Animal Model on Subsequent Release of Payload Release in Vivo The objective of this experiment is to firstly to demonstrate that the relevant peptide can be released in a time dependant, pulsatile manner, when ultrasound is applied to the kidney (cortical region) of the pig animal model. Secondly, uptake of the labelled payload into surrounding tissue cells is observed. The test system comprises one animal, a description of which including blood collection, loading conditions, anaesthesia and sample collection conditions has been provided in the above Examples, with the following modifications. After 30 mins, ultrasound is applied to the cortical region of the right kidney at 6W/cm 2 (pulsed wave; 35%) (RICH-MAR CRM-1 machine fitted with a IMHz head), with two sequential and different sets of ultrasound conditions. The 1 st set consists of 5 cycles each of 30 seconds with a 4 min 30 sec rest between cycles, while and the 2 nd set consists of 5 cycles each of 1 minute with a 4 min rest between cycles. The surface of this area is liberally covered with Alpha Lube gel (Ultrasonic Scanning gel, BCF Technology Ltd) before application of the head. Samples are collected at the time periods shown, and analysed using flow cytometry, where cells counts of the loaded cell population are assessed. The treated and non-treated kidneys are fixed in 4% paraformaldehyde for subsequent wax embedding. Sections (10 μM) are viewed under the fluorescent microscope to assess the amount of labelled peptide localised inside the cells. Results 12 Figure 12A demonstrates that the loaded vehicle retains a high level of stability in vivo, which remains constant prior to ultrasound application. Across the duration of 2 minutes treatment there does not appear to be any ultrasound mediated effect. However, after the 4 th burst for 30 seconds, there is a stepwise decrease in loaded cell number and this cumulative effect continues. This indicates that the payload is being released in a pulsatile, stepwise or discontinuous manner. Figure 12B illustrates the ultrasound mediated localisation of FITC-labelled TAT in the treated kidney compared to the control organ. This clearly exhibits an enhanced localisation and uptake at the treated area, indicating that the peptide is released at the site of ultrasound treatment, and subsequently taken up by the cells in close proximity. The following three Examples demonstrate optimal electrosensitisation and loading of FITC-labelled HIV-TAT into murine erythrocytes, ultrasound mediated release of payload in whole circulating mouse blood in vitro, and ultrasound mediated pulsatile release and localised uptake in a murine in vivo model. Example 13. Production of an Optimally Electrosensitised Murine Erythrocyte Which is Loaded with the Peptide HIV-TAT Whole blood from mouse is collected in heparinised containers and the cells washed as described in the general protocols set out in Example 1 (second procedure) ofPCT/GBOO/03056. Optimal conditions for cell concentration, electrosensitisation voltage and pulse number are established. Cells are washed once in PBS-Mg at 700g for 5»min, and once in buffer 1 at 700g for 5 mins. The cells are retained as a packed cell volume and fluorescein-labelled HIV TAT (Alta Biosciences, Edgbaston, Birmingham) is added to the packed cell volume at a concentration of 0.04mg/ml (expressed as mg/ml peptide to 7 x 10 8 cells/ml)' Mixtures are placed in dialysis tubing (1 OOODa MW tubing, Spectro-Por, Spectrum Inc.,). Cells are then dialysed against buffer 2 for one hour at 4°C. Membranes are then placed into mBAX at 37°C, and dialysed for one hour. Cells are harvested from the dialysis membranes and washed three times in mBAX buffer at 170g for 15 minutes at room temperature. Cells are resuspended at 7 x 10 in mBAX and stored at 4°C overnight. Cell recovery and sensitivity are assessed as markers by which conditions for an optimally sensitised and loading murine erythrocyte is obtained. Loading of peptide into the erythrocyte is monitored by analysis on flow cytometry where the uptake of the fluorescent peptide is indicated by a shift to the right on the flow cytometry profiles. Results 13 The results are shown in Figures 13 A and 13B, where a cell density of 10 x 10 s and 1 pulse at 1.45kV elicit optimal cell recoveries and ultrasound sensitivity. Peptide loading into the erythrocyte is shown a shift to the right on the flow cytometry profiles (Figure 13C). The progressive shift the right is indicative of peptide uptake by the sensitised carrier vehicle The peptide-loaded cells are shown to be preferentially sensitive to low intensity ultrasound (100%) lysis when treated with ultrasound at 3 W/cm 2 and at IMHz using the TMM system as described in WO 01/07011). These results demonstrate that HIV-TAT may be dialysis loaded into electrosensitised mouse erythrocytes, producing a carrier which is sensitive to ultrasound. Example 14. Ultrasound Mediated Release of Payload in Whole Circulating Mouse Blood in Vitro The objective of this experiment is to demonstrate that the relevant peptide could be released by ultrasound from a loaded mouse erythrocyte in an in vitro circulating model, at 37 C, 1.3 cm from the ultrasound probe, spiked into whole blood. From this, ultrasound parameters may be established for further use in an in vivo system. Mouse erythrocytes, electrosensitised and dialysis loaded with HIV-TAT are spiked (2.5%) into whole blood of the same animal. A 3ml sample is then applied to the circulating phantom model at 4.5 - 6W/cm 2 (pulsed wave; 35%) for 15 min, and lOOμl samples collected for the circulating system every 5 min. Any ultrasound mediated decrease in loaded erythrocytes is demonstrated by loss of cells on the flow cytometer. Haemoglobin levels in the supernatants of the collected samples are assessed at Abs 5 o nm on the spectrophotometer. Results 14 Figure 14A demonstrates that under the parameters used, an ultrasound intensity of 4.5 W/cm 2 confers negligible effects on the number of loaded cells in whole blood. At 5-6W/cm 2 a decrease in the number of loaded cells occurs after 2 min. Figure 14B demonstrating haemoglobin release at the various ultrasound intensities shows that release of this cell lysis marker mirrors the loss of labelled cells, showing that these cells are being targeted by ultrasound. These results show that in the in vitro circulating system, the loaded vehicle spiked into whole blood, is sensitive to ultrasound. Example 15. Ultrasound Mediated Release of Payload in vivo from Mouse Erythrocytes The objective of these experiments is to demonstrate firstly that the relevant peptide payload can be released in a pulsatile manner from the vehicle, in the context of an in vivo murine environment. On release from the vehicle, it has been demonstrated that the peptides are capable of trafficking into target cells. In terms of exploitation in this invention, the functionality of the peptide is used to traffic into and beyond the vascular endothelium. Therefore, subsequent uptake of fluorescently-labelled peptide into endothelial cells can be investigated in an in vivo model. The test system comprises two male Swiss To mice (8-12 weeks). Anaesthesia is induced by inhalation with isofluorane and maintained under 2 % isofluorane with a flow rate of 21 oxygen/min). Administration of loaded packed cells (200 μl) and sampling (lμl) is carried out from the tail veins (one for each). In one of the subjects, after 15 min, ultrasound is applied directly to the cortical region of the kidney at 6W/cm 2 (pulsed wave; 35%>) (RICH-MAR CRM-1 machine fitted with a IMHz head, for 2 x 5 min bursts, with a 5 minute rest between bursts. The surface of this area is liberally covered with Alpha Lube gel (Ultrasonic Scanning gel, BCF Technology Ltd) before application of the probe head. Samples are collected at the time periods shown, and analysed using flow cytometry, where cell counts of the loaded cell population are assessed. The treated and non-treated kidneys are fixed in 4%> paraformaldehyde for subsequent wax embedding. Sections (10 μM) are viewed under the fluorescent microscope to assess the amount of labelled peptide localised inside the cells. Results 15 Figure 15 A demonstrates that a clear increase in % loaded cells coincides with administration of loaded vehicle into the animal. In the control animal, to which no ultrasound is applied, a gradual decline in labelled cell number is observed. In contrast, the effect of ultrasound treatment on loaded cells in vivo is pronounced, and a dramatic decrease is shown between following both 5 minute bursts of ultrasound treatment at 6W/cm , pulsed wave; 35%. Figure 15B illustrates the ultrasound mediated localisation of FITC-labelled TAT in the treated kidney compared to the control organ. Less fluorescent staining is evident in the kidney to which no direct application of ultrasound is carried out. However, in the tissues which ultrasound is applied, strong fluorescence is evident, clearly exhibiting enhanced localisation and uptake at the treated area. This indicates that the peptide released at the site of ultrasound treatment,trafficks across cell membranes into neighbouring endothelial cells. This phenomenon may be exploited to enable uptake of payload conjugates by target tissues following ultrasound-mediated release from the erythrocyte vehicle. Example 16. Uptake of TAT, Oligonucleotide and TAT-OIigonucleotide by the Lining of Blood Vessels In this series of experiments the objective is to determine whether or not TAT, a candidate oligonucleotide (Scaggiante et al, Eur. J. Biochem., 252, 207-215) and a TAT-oligonucleotide conjugate are taken up by the inner lining of blood vessels. In order to do so, TAT is labelled with FITC (fluorescein isothiocyanate) (Alta Biosciences UK), the oligonucleotide having a sequence 5' TGT TTG TTT GTT TGT TTG TTT GTT TGT 3' (MWG Biotech, UK) is labelled with biotin and the conjugate is co-labelled with FITC on the peptide and biotin on the oligonucleotide (Alta Biosciences, UK). Samples of each molecular species ([TAT] = 200μg/ml; [oligonucleotide] = 50μg/ml and [TAT-oligonucleotide conjugate] = 250μ/ml) are placed in contact with the inner surface of rabbit aorta at room temperature for 3 Omin. Following incubation, each section of aorta is washed 3 times in phosphate buffered saline and then placed in a vial containing 4%> (w/v) paraformaldehyde solution. Paraffin wax sections of each sample are prepared and viewed directly using a fluorescent microscope to detect the presence of the peptide (FITC) either alone or as a molecular partner in the conjugate. Detection of the oligonucleotide, either alone or as a partner in the conjugate is accomplished by incubating sections in 0.3% H 2 O 2 for 3 Omin. at room temperature. After rinsing for lOmin. in water, sections are incubated with Elite ABC complex (avidin-peroxidase) (Vectastain, USA) for 90min at 37°C. Slides are washed in phosphate buffered saline for 3 min. and subsequently incubated in peroxidase substrate (3,3'-diaminobenzidine [DAB]) for 5min. at room temperature. The reaction is terminated with water and sections are examined using a light microscopy. Results 16 The data obtained from these experiments are shown in Figures 16A and 16B. Clear fluorescent staining on the inner vessel surface is observed under fluorescence microscospy in aorta samples which are in contact with either TAT-oligonucleotide conjugate (Figure 16A, Panel B) or TAT alone (Figure 16 A, Panel C). No fluorescence is detected in samples which are incubated with oligonucleotide alone (Figure 16A, Panel A). These results demonstrate that both TAT and TAT-oligonucleotide are taken up by the inner lining of the aorta. In addition, clear staining for biotin (the presence<bf oligonucleotide) is present on the inner vessel surface in samples which are in contact with oligonucleotide alone and TAT-oligonucleotide conjugate (Figure 16 A, Panel D & Figure 16B Panel E, respectively). No staining is evident in sections that are in contact with TAT alone (Figure 16B, Panel F). These results demonstrate that oligonucleotide is taken up by the inner lining of the vessel which are in contact with oligonucleotide alone or with TAT-oligonucleotide conjugate. In overall terms the results demonstrate the co-existence of the TAT peptide and oligonucleotide partners in tissues placed in contact with the conjugate (Figure 16A, Panel B & Figure 16B, Panel E) and this demonstrates functionality of the peptide in terms of oligonucleotide carriage into tissue. Example 17. Uptake of Oligonucleotide and TAT-Oligonucleotide Conjugate by the Inner Surface of Aorta Following Ultrasound-Mediated Release from Human Erythrocytes In this series of experiments it is decided to load the oligonucleotide and the TAT-oligonucleotide into human erythrocytes. The loaded erythrocytes are then exposed to ultrasound and the resulting lysates placed in contact with the inner surface of a blood vessel. Demonstrating uptake of the conjugate into the vessel inner lining by detecting the presence of both TAT and oligonucleotide partners indicates functionality of the TAT partner in terms of payload carriage to that tissue. Human erythrocytes (7x10 8 cells/ml in PBS) are sensitised and loaded with lmg/ml oligonucleotide (labelled with biotin) and TAT-oligonucleotide conjugate (labelled on the peptide with FITC and on the oligonucleotide with biotin) as described previously (Example 6). Following washing, loading of the conjugate is confirmed by flow cytometry as shown in Figure 17. A shift in the population to the right indicates loading with the FITC label on the peptide partner of the conjugate. lOOμl aliquots of cells are exposed to ultrasound (3W/cm 2 , 36 seconds at IMHz) using the tissue mimicking system as described for Example 6. Lysates resulting from ultrasound treatment are recovered and incubated at room temperature together with the inner surface of rabbit aorta for lh. Following incubation, tissues are washed three times in PBS and samples are treated as described in the previous example above. Sections are examined using fluorescence microscopy to detect the presence of TAT. Sections of aorta are also stained for biotin (the presence of oligonucleotide) and examined using light microscopy as described above. Results 17 The results following examination of sections using fluorescence microscopy are shown in Figure 18 and they demonstrate clear fluorescent staining on the inner surface of vessel which is in contact with lysates from cells loaded with the conjugate (Figure 18, Panel B). No fluorescent staining is observed in control tissue which is in contact with phosphate buffered saline (Figure 18, Panel A) or in tissues which are in contact with oligonucleotide. These results demonstrate that the TAT partner in the conjugate is taken up by the inner lining of the vessel. The results also demonstrate that the TAT exhibits functionality following ultrasound-mediated release from human erythrocytes. The results obtained following staining of the vessel samples for the presence of biotinylated oligonucleotide are shown in Figure 19 and they again show clear staining for oligonucleotide in samples of tissue which are in contact with both the oligonucleotide- and TAT-oligonucleotide conjugate-containing lysates (Figure 19A, Panels A & B, respectively). In samples of tissue which are placed in contact with the intact vehicle, loaded with either oligonucleotide or conjugate, no staining for oligonucleotide is detected (Figure 19B, C & D, respectively). The results demonstrate that either oligonucleotide or conjugate, released from the loaded erythrocytes using ultrasound, is taken up by the tissues. They also confirm that the oligonucleotide co- resides with the TAT partner of the conjugate in tissues that are in contact with ultrasound-derived lysates of cells loaded with that conjugate. The results demonstrate that the TAT remains functional in terms of uptake by tissues following ultrasound- mediated release from the erythrocyte vehicle. Example 18. Ultrasound-Mediated Deposition of TAT-Oligonucleotide Conjugate in Mouse Kidney in Vivo In this series of experiments mouse erythrocytes are loaded with oligonucleotide (biotinylated) and TAT-oligonucleotide (FITC on the peptide and biotin on the oligonucleotide). Recipient animals are anaesthetised using 2%o isofluorane in a 2L/min O 2 carrier. The preparations are injected into recipient animals (50-200μl) through the tail vein and allowed to circulate for 5 min. The kidney is surgically exposed through the abdomen and ultrasound gel is placed over the target kidney to mediate contact with the ultrasound head. Treatment consisted of exposing the target kidney to IMHz ultrasound at 6W/cm 2 and using pulsed ultrasound at 35% continuous wave for 4 min. Following treatment, both treated and untreated kidneys are harvested from the animal and placed in a 4% paraformaldehyde solution. Paraffin wax sections are prepared as described above and sections are either observed directly for fluorescence (presence of the TAT partner) or stained for biotin (presence of oligonucleotide). The latter are then viewed using light microscopy. Results 18 In both cases, the non-target kidney shows no staining for either fluorescent signal from the TAT partner in the conjugate or for biotinylated oligonucleotide, indicating a lack of deposition in the non-treated organ. In the treated organ from the animal receiving the oligonucleotide loaded erythrocytes, little or no staining is evident. However, in the treated organ from the animal injected with the conjugate, a clear fluorescent signal is evident and this indicated the presence of TAT. In addition, sections from this organ also exhibit a positive signal for biotin and this indicates the co-deposition of oligonucleotide in the tissues. The results demonstrate that treatment with ultrasound facilitates deposition of conjugate in the target organ and that conjugate is retained at the target side as a result of TAT functionality. The latter facilitates retention of oligonucleotide at the target and this is confirmed by an absence of oligonucleotide in treated kidney from the animal injected with oligonucleotide- loaded erythrocytes. Each of the applications and patents mentioned above, and each document cited or referenced in each of the foregoing applications and patents, including during the prosecution of each of the foregoing applications and patents ("application cited documents") and any manufacturer's instructions or catalogues for any products cited or mentioned in each of the foregoing applications and ' patents and in any of the application cited documents, are hereby incorporated herein by reference. Furthermore, all documents cited in this text, and all documents cited or referenced in documents cited in this text, and any manufacturer's instructions or catalogues for any products cited or mentioned in this text, are hereby incorporated herein by reference. Various modifications and variations of the described methods and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in molecular biology or related fields are intended to be within the scope of the following claims.

Claims: CLAIMS 1. A method of preparing a red blood cell vehicle suitable for delivering an agent to a target site in a vertebrate, the method comprising the steps of: (a) providing a red blood cell; and (b) loading the red blood cell with an agent-MTS conjugate. 2. A method according to Claim 1 , which further comprises the step of sensitising the red blood cell, whether before or after the loading step, to render the red blood cell more susceptible to disruption by exposure to a stimulus than an unsensitised red blood cell. 3. A method of preparing a red blood cell vehicle suitable for delivering an agent to a target site in a vertebrate, the method comprising the steps of: (a) providing a red blood cell loaded with an agent-MTS conjugate; and (b) sensitising the red blood cell. 4. A method of preparing a red blood cell vehicle suitable for delivering an agent to a target site in a vertebrate, the method comprising the steps of: (a) providing a sensitised red blood cell; and (b) loading the red blood cell with an agent-MTS conjugate. 5. A method for delivering an agent to a target site in a vertebrate, comprising the steps of: (a) providing a red blood cell; (b) loading the red blood cell with an agent-MTS conjugate; (c) sensitising the red blood cell to render it more susceptible to disruption than an unsensitised red blood cell; (d) introducing the red blood cell into a vertebrate; and (e) causing the agent-MTS conjugate to be released from the sensitised red blood cell by applying energy to the sensitised red blood cell; in which steps (b) and (c) may be performed in any order. 6. A red blood cell vehicle suitable for delivery of an agent to a vertebrate, the red blood cell comprising agent-MTS conjugate. 7. A red blood cell vehicle according to Claim 6, in which the red blood cell is sensitised so that it is rendered more susceptible to disruption by exposure to a stimulus than an unsensitised red blood cell. 8. A method according to any of Claims 2 to 5, or a red blood cell according to Claim 7, in which the red blood cell is sensitised by applying an electric field to the red blood cell. 9. A method or red blood cell according to Claim 8, in which the electric field has a field strength of from about O.lkVolts/cm to about 10 kVolts/cm under in vitro conditions. 10. A method or a red blood cell according to Claim 8 or 9, in which the red blood cell is sensitised by application of an electric pulse for between lμs and 100 milliseconds. 11. A method according to any of Claims 2 to 5 and 8 to 10, or a red blood cell according to any of Claims 7 to 10, in which the sensitised red blood cell is capable of being disrupted by exposure to ultrasound. 12. A method or a red blood cell according to Claim 11, in which the ultrasound is selected from the group consisting of diagnostic ultrasound, therapeutic ultrasound and a combination of diagnostic and therapeutic ultrasound. 13. A method or a red blood cell according to Claim 11 or 12, in which the applied ultrasound energy source is at a power level of from about 0.05W/cm 2 to about lOOW/cm 2 . 14. A method or a red blood cell according to any preceding claim, in which the red blood cell vehicle is pre-sensitised so that it is capable of being loaded with a larger amount of agent than a red blood cell which has not been pre-sensitised. 15. A method according to Claim 14, in which the pre-sensitisation comprises exposing the red blood cell to an electric field and/or ultrasound. 16. A method or red blood cell according to any preceding claim, in which the agent-MTS conjugate comprises a membrane translocation sequence enabling the agent to cross the plasma membrane of a cell. 17. A method or red blood cell according to any preceding claim, in which the agent-MTS conjugate comprises a fusion protein, in which the polypeptide is fused to a membrane translocation sequence. 18. A method or red blood cell according to any preceding claim, in which the agent-MTS conjugate comprises a membrane translocation sequence selected from: HIV- 1 -trans-activating protein (Tat), Drosophila Antennapedia homeodomain protein (Antp-HD), Herpes Simplex- 1 virus VP22 protein (HSV-VP22), signal-sequence- based peptides, Transportan and Amphiphilic model peptide, homologues of the foregoing, and fragments, variants and mutants having membrane translocational activity. 19. A method or red blood cell according to any preceding claim, in which the agent-MTS conjugate comprises the membrane translocation sequence GRKKRRQRRRPPQC, RQIKIWFQNRRMKWKK or RQIKIWFQNRRMKWKKC. 20. A method or red blood cell according to any preceding claim, in which the agent is selected from a group consisting of a biologically active molecule, a protein, a polypeptide, a peptide, a nucleic acid, an oligonucleotide, a peptide nucleic acid (PNA), a virus-like particle, a nucleotide, a ribonucleotide, a deoxyribonucleotide, a modified deoxyribonucleotide, a heteroduplex, a nanoparticle, a synthetic analogue of a nucleotide, a synthetic analogue of a ribonucleotide, a modified nucleotide, a modified ribonucleotide, an amino acid, an amino acid analogue, a modified amino acid, a modified amino acid analogue, a steroid, a proteoglycan, a lipid, a carbohydrate, and mixtures, fusions, combinations or conjugates of the above. 21. A method or red blood cell according to any preceding claim, in which the agent is conjugated to, fused to, mixed with or combined with an imaging agent. 22. Use of a red blood cell prepared according to any of Claims 1 to 5 and 8 to 21, or a red blood cell according to Claim 6 or 7, in the preparation of a medicament for delivery of an agent to or at a target site. 23. Use of a red blood cell prepared according to any of Claims 1 to 5 and 8 to 21 , or a red blood cell according to Claim 6 or 7, for the delivery of one or more agents to a vertebrate. 24. Use according to Claim 23, in which the agent is actively released from the red blood cell vehicle by application of a stimulus to disrupt the red blood cell vehicle. 25. A kit comprising a red blood cell prepared according to any of Claims 1 to 5 and 8 to 21, or a red blood cell according to Claim 6 or 7, an agent comprising a membrane translocation sequence suitable for loading into said red blood cell and packaging materials therefor. 26. A pharmaceutical composition comprising a red blood cell prepared according to any of Claims 1 to 5 and 8 to 21, or a red blood cell according to Claim 6 or 7, together with a physiologically compatible buffer. 27. A method of loading a red blood cell with an agent, the method comprising the steps of: (a) providing a red blood cell; and (b) exposing the red blood cell to an agent-MTS conjugate. 28. Use of a membrane translocation sequence in a method of delivery of an agent to a vertebrate, in which the method comprises the steps of: (a) providing an agent to be delivered; (b) joining the agent to a membrane translocation sequence; and (c) loading the agent joined to the membrane translocation sequence into a red blood cell vehicle. 29. A method of immunisation of an animal with an antigen, the method comprising the steps of: (a) providing a red blood cell; (b) loading the red blood cell with an antigen; (c) introducing the red blood cell into a vertebrate; and (d) causing the agent to. be released from the red blood cell. 30. A method according to Claim 29, in which the red blood cell is sensitised, more preferably, electrosensitised, to render the red blood cell more susceptible to disruption by exposure to a stimulus than an unsensitised red blood cell. 31. A method according to Claim 29 or 30, in which the red blood cell is disrupted by exposure to ultrasound. 32. A method according to any of Claims 29 to 31 , in which steps (c) and/or (d) are repeated. 33. Use of a membrane translocation sequence to load an agent into a red blood cell. 34. A method of producing a red blood cell suitable for delivery of a polypeptide to a vertebrate, the method comprising the steps of: (a) providing a transgenic animal carrying and expressing a transgene encoding a fusion protein comprising the polypeptide and a membrane translocation sequence (MTS); (b) obtaining a red blood cell containing the fusion protein from the animal; and (c) sensitising the red blood cell sensitising the red blood cell to render it susceptible to disruption by an energy source. . 35. A method of producing a red blood cell suitable for delivery of a polypeptide to a vertebrate, the method comprising the steps of: (a) providing a red blood cell containing a polypeptide, the red blood cell being derived from a transgenic animal carrying and expressing a transgene encoding a fusion protein comprising the polypeptide and a membrane translocation sequence (MTS); and (b) sensitising the red blood cell to render it susceptible to disruption by an energy source. 36. A method for the delivery of a polypeptide to a vertebrate, the method comprising the steps of: (a) providing a transgenic animal carrying and expressing a transgene encoding a fusion protein comprising the polypeptide and a membrane translocation sequence (MTS); (b) obtaining a red blood cell containing the fusion protein from the animal; (c) sensitising the red blood cell to render it susceptible to disruption by an energy source; (d) introducing the sensitised red blood cell to a vertebrate; and (e) exposing the vertebrate, or a part of it, to an energy source at a level sufficient to disrupt the sensitised red blood cell. 37. A method of producing a polypeptide agent-MTS conjugate, the method comprising the steps of: (a) isolating a red blood cell from a transgenic animal carrying and expressing a transgene encoding a fusion protein comprising the polypeptide and a membrane translocation sequence (MTS); (b) sensitising the red blood cell to render it susceptible to disruption by an energy source; (c) exposing the red blood cell to an energy source sufficient to disrupt the sensitized red blood cell; and (d) isolating the fusion protein to provide the polypeptide agent-MTS conjugate. 38. A method according to any of Claims 34 to 37, in which the transgenic animal is selected from the group consisting of: mouse, rat, rabbit, sheep, goat, cow, and pig. 39. A method according to any of Claims 34 to 38, in which the polypeptide is expressed under the control of a β-globin promoter or enhancer. 40. A method according to any of Claims 34 to 39, in which the polypeptide is expressed under the control of a β-globin Locus Control Region (LCR).

Inventors: CRAIG ROGER, CRAIG, ROGER
Applicants: CRAIG ROGER, GENDEL LTD, CRAIG, ROGER, GENDEL LIMITED
Application year: 2001
Main classification: A61K38
IPC codes: A61K9, A61K35, A61K38, C07K14, C12N5, C12N15