EP1015036A1 - Improvements in or relating to expression of immunogenic substances - Google Patents

Abstract

Disclosed is a method of regulating the expression in a human or animal subject of a nucleic acid sequence encoding a polypeptide which is immunogenic in the subject; the method comprising introducing into the subject a cell comprising the nucleic acid sequence encoding the immunogenic polypeptide, said sequence being operably linked to a drug-regulatable promoter; and altering the concentration of regulatory drug to which the cell is exposed.

Description

Title: Improvements in or Relating to Expression of Immunogenic Subst.ances

Field of the Invention

This invention relates, inter alia, to methods of regulating the expression of nucleic acid sequences in eukaryotic cells and, in particular, to a method of regulating the expression of immunogenic polypeptides.

Background of the Invention

Unlike antibody molecules some cells, such as T cells, can migrate actively and efficiently through micro vascular walls, allowing them to penetrate into tissues (e.g. the core of a solid tumor) before they exert their cytolytic function. Ex vivo expansion and re-infusion of autologous tumor-reactive T cells is being explored as an experimental approach to cancer therapy. However, circulating T cells from peripheral blood lack specificity for tumor antigens (1) and it is often impractical or impossible to obtain sufficient numbers of tumor infiltrating lymphocytes (2). To overcome these problems new approaches have been developed whereby antibody specificity can be combined with the efficient trafficking properties and effector functions of T cells. Several reports have demonstrated the feasibility of transfecting cultured T cells with genes encoding chimeric receptors in which single-chain antibody domains (scFv) are linked to different signalling portions of the TCR/CD3/ " complex as a means to target T cells towards native antigens (3-6). However it is apparent that the long-term clinical success of this "T-body" approach could depend on the development of solutions to a number of problems.

Although different strategies have been developed for regulating transgene expression in eukaryotic cells (7), the tetracycline-regulatable system (TRS) avoids the problems related to many of these systems by offering substantial regulation of transgene expression in response to concentrations of tetracycline that cause little or no toxicity in mammalian cells (8, 9).

Miller & Whelan (1997 Hum. Gene Therapy 8, 803-815) have recently reviewed progress towards the development of regulatable vectors for gene therapy. Among the vectors described are those using the TRS.

In the TRS originally described by Gossen & Bujard (1992 Proc. Natl. Acad. Sci. USA 89, 5547-5551), the tetracycline repressor protein was fused to the Herpes Simplex Virus (HSV) VP16-activating domain, to create a chimeric tetracycline-repressible transactivating (tTA) polypeptide, which binds to DNA comprising the tet operator sequence, causing transcriptional activation of coding sequences downstream of the operator. The presence of tetracycline or analogues thereof (such as doxycycline, anhydrotetracycline, minocycline and oxytetracycline) inhibits this transcriptional activation, as these compounds bind to the tTA polypeptide, altering its conformation and prevent its binding to the tet operator sequence.

Variants of the original TRS have now been described (WO 96/01313) in which a mutant form of the tet repressor protein binds to DNA in the presence, but not in the absence, of tetracycline or its analogues. Thus, in these systems, expression of a tet operator-linked gene is positively regulated in the presence of tetracycline or its analogues.

However, there .are a number of difficulties and/or uncertainties relating to the use of the TRS. For example, Cooke et al, (1997 J. Gen. Virol. 78, 381-392) found that the TRS could not be used to regulate expression of the nef gene in the human T cell line Jurkat E6-1. Further, the HSV VP16 domain is associated with a toxic "squelching" effect, such that high levels of regulation have not previously been obtainable. A recent review (Miller & Whelan 1997 Hum. Gene Ther. 8, 803-815) has stated that the TRS "may not be applicable to all cell lines" (citing the work of Ackland-Berglund & Leib 1995 BioTechniques 18, 196-200; and Howe et al. , 1995 J. Biol. Chem. 270, 14168-14174).

Summary of the Invention

In a first aspect the invention provides a method of regulating me expression in a human or animal subject of a nucleic acid sequence encoding a polypeptide which is immunogenic in the subject; the method comprising introducing into the subject a cell comprising the nucleic acid sequence encoding the immunogenic polypeptide, said sequence being operably linked to a drug-regulatable promoter; and altering the concentration of a regulatory drug to which the cell is exposed.

Desirably the cell is one which is capable of undergoing extravasation, such as a leukocyte. Particularly preferred cell types are lymphocytes (B or T lymphocytes), monocytes or macrophages.

The term "drug-regulatable promoter" is intended to refer to portions of nucleic acid (e.g. enhancers, or more preferably, promoters) which regulate the expression of coding regions of nucleic acid, in response to the presence of a substance (the "regulatory drug") exogenous to the cell. The regulatory drug is such that it causes modification of the activity of the drug-regulatable promoter at a concentration which is non-lethal to the cell in which the nucleic acid is present. Preferably the action of the regulatory drug is substantially specific for the drug-regulatable promoter such that, if it is necessary to administer the drug to the patient it will not cause widespread effects on other tissues or cell types. Such specificity is most readily conferred by use of a synthetic regulatory drug and a corresponding drug-regulatable promoter which is not normally present in the subject. The drug-regulatable promoter may be directly drug-regulatable, or more typically, indirectly drug-regulatable. An example of a system comprising an indirectly drug-regulatable promoter is a system in which a drug-regulatable operator exerts a regulatory effect on a promoter, which effect depends on the concentration of regulatory drug.

A number of suitable drug-regulatable promoters and corresponding regulatory drugs are known (see for example, Miller & Whelan, Hum. Gene Ther. 8, 803-815), and include promoters regulated by glucocorticoid steroids, sex hormone steroids, lipopolysaccharide (LPS) and isopropylthiogalactoside (IPTG). Suitable concentrations of the corresponding drugs will be apparent to those skilled in the art. As a general guide, when culturing cells in vitro in the presence of steroids a preferred concentration for the steroid drug is InM to ImM. When the steroid drug is administered in vivo to a human or animal subject, the dosage is desirably adjusted to achieve a serum concentration beween about 0.05 and 10.0 μg/Kg. When culturing cells in the presence of IPTG, the dosage is preferably in the range of lμM to lOOmM, more preferably in the range lOOμM to ImM. When culturing cells in the presence of LPS the dosage is conveniently in the range 0.1 nM to ImM, more desirably in the range lOOμM to ImM

Another system, particularly useful in applying the method of the present invention, is the tetracycline-regulatable system (TRS), which has been extensively studied and several variants thereof have been developed, making the TRS extremely adaptable. For example, in the absence of tetracycline the wild-type bacterial tet repressor protein causes negative regulation of bacterial tetracycline-resistance genes: tetracycline binds to the repressor protein and prevents it from binding to the tet operator DNA sequence, thus allowing expression of the resistance genes. Conversely, a chimeric polypeptide comprising part of the tet repressor and the HSV VP16 transactivating domain causes positive regulation (transcriptional activation) of coding sequences. Other transactivating domains are known to those skilled in the art (e.g. amino acid residues 753-881 of GAL4; amino acid residues 399-499 of CTF/NF1; and those from ITF1 or ITF2), and these may conceivably be used to form a chimeric tetracycline-sensitive or other regulatory drug-sensitive polypeptide.

In addition to the chimeric polypeptides described in the prior art (e.g. Gossen & Bujard 1992, cited above) which comprise a portion of the wild type tet repressor protein, other polypeptides are known (disclosed in WO 96/01313) which comprise a mutated form of the tet repressor protein, which binds to the tet operator sequence in the presence, but not in the absence, of tetracycline.

Accordingly the method of the invention is such that in some embodiments the presence of tetracycline will serve to inhibit expression of a tetracycline operator (tet O) - linked coding sequence (i.e. the sequence which encodes the immunogenic polypeptide), whilst in other embodiments the presence of tetracycline will serve to enhance expression of the tet O-linked sequence.

Those skilled in the art will appreciate that a number of analogues of tetracycline are known, which can readily be substituted for tetracycline in the embodiments described above. Indeed, certain analogues may actually be preferred to tetracycline, as they may have higher binding affinities (for the tetracycline-sensitive polypeptide) and so exert a regulatory effect at lower concentrations. A tetracycline analogue may be understood to refer to a molecule which is structurally related to tetracycline and which binds to the Tet repressor with a K^ of at least about 10⁶ M-¹, preferably 10⁹ M-¹ or greater. Preferred analogues are doxycycline and anhydrotetracycline. Other analogues include minocycline, oxy tetracycline, chlorotetracycline, epioxytetracycline and cyanotetracycline. Other analogues of tetracycline are described by Hlavka & Boothe ("The Tetracyclines" in "Handbook of Experimental Pharmacology 78, Blackwood et al, (eds.) Springer Verlag 1985).

The tet operator (tet O) sequence is now well-known to those skilled in the art. For a review, the reader is referred to Hillen & Wissmann (1989) in Protein-Nucleic Acid Interaction. "Topics in Molecular and Structural Biology", eds. Saenger & Heinemann, (Macmillan, London), Vol. 10, pp 143-162. Typically the nucleic acid sequence encoding the immunogenic polypeptide will be placed downstream of a plurality of tet O sequences: generally 5 to 10 such tet O sequences are used, in direct repeats.

Conveniently, the tet 0 sequences will be fused substantially adjacent (i.e. within lOObp, preferably within 50 bp) to a "minimal" (i.e. enhancerless) eukaryotic promoter (such as the minimal CMV immediate early promoter, described previously [Gossen & Bujard 1992 Proc. Natl. Acad. Sci. USA 89, 5547]), such that binding of a transactivating tetracycline- sensitive polypeptide to the tet O sequence will cause enhanced expression of the tet O- linked coding sequence.

Constructs particularly suitable for introduction of the sequence encoding the immunogenic polypeptide and/or the drug-regulatable promoter are well-known and have been disclosed previously (e.g. Baron et al, 1995 Nucl. Acids Res. 23, 3605-3606; Schultze et al, 1996 Nature Biotechnology 14, 499-503). A particularly preferred construct (where tetracycline. or an analogue thereof, is the regulatory drug), found to allow for very high levels of regulation of expression of tet O-linked sequences, is disclosed in co-pending UK Patent Application Number 9718873.4, and by Mohammadi et al. , (1997 Gene Therapy 4, 991-997; 1998 Gene Therapy 5, 76-84) and by Alvarez-Vallina et al, (1997 J. Immunol. , 5889-5895).

The method of the invention finds application in a number of fields, but particularly in the field of gene therapy. Thus, whilst the method of the invention may be useful in regulating the expression of any polypeptide immunogenic to the subject in question, it is desirably applied to the regulation of the expression of one or more therapeutic polypeptides, and especially useful in regulating the expression of a polypeptide in a human subject. Those skilled in the art are familiar with the wide range of polypeptides which have potential usefulness when expressed as therapeutic polypeptides in patients by means of gene therapy techniques, but which may present difficulties because of their immunogenicity when expressed in for example, a human patient because they are not normal human proteins. Examples of such immunogenic polypeptides include proteins from other sources (e.g. from plants, animals, fungi, bacteria, yeasts and the like), and chimeric polypeptides which comprise portions of proteins from non-human sources, or even chimeric polypeptides which create novel fusions of human proteins or parts thereof and which are therefore immunogenic in a human subject.

Another category of potentially immunogenic therapeutic proteins are those which are not expressed or are not expressed in a functional form due to a genetic defect. Examples of such polypeptides include growth factors and blood clotting factors, hormones, neurotransmitters, enzymes, apolipoproteins, receptors, drugs, oncogenes, tumor antigens, tumor suppressors, structural proteins, viral antigens, parasitic antigens and bacterial antigens.

Specific examples of these compounds include CFTR, insulin, adenosine deaminase, pro insulin, growth hormone, dystrophin, androgen receptors, insulin-like growth factor I, insulin-like growth factor II, insulin-like growth factor binding proteins, epidermal growth factor, TGF-α:, TGF- 3, PDGF, angiogenesis factors (acidic fibroblast growth factor, basic fibroblast growth factor and angiogenin), matrix proteins (Type IV collagen, Type VII collagen, laminin), phenylalanine hydroxylase, tyrosine hydroxylase, oncogenes (ras, fos, myc, erb, src, sis, jun), E6 or E7 transforming sequence, p53 protein, Rb gene product, cytokine receptor, IL-1, IL-6, IL-8, viral capsid protein, and proteins from viral, bacterial and parasitic organisms which can be used to induce an immunologic response, and other proteins of useful significance in the body. In principle, any compounds for which there is an available nucleic acid sequence for the protein or polypeptide to be incorporated can be used according to the invention.

Another category of therapeutic agents useful according to the invention includes immunoactive agents, i.e. , agents which combat viral infections or production by activating an immune response to the virus. Such agents include but are not limited to cytokines against viruses in general (Biron, 1994, Curr. Opin. Immunol. 6:530); soluble CD4 against SIV (Watanabe et al. , 1991, Proc. Nat. Aca. Sci. 88: 126); CD4- immunoglobulin fusions against HIV-1 and SIV (Langner et al. , 1993, Arch. Virol. 130:157); CD4(81-92)-based peptide derivatives against HTV infection (Rausch et al. , 1992, Biochem. Pharmacol. 43: 1785); lympho-cytotoxic antibodies against HIV infection (Szabo et al. , 1992, Acta. Virol. 38:392); IL-2 against HIV infection (Bell et al. , 1992, Gin Exp. Immunol. 90:6); and anti-T cell receptor antibodies against viruses in general (Newell et al., 1991, Ann. N.Y. Aca. Sci. 636:279).

The subject is typically a human patient, but the method of the invention is potentially applicable to any mammal, such as domesticated mammals or farm animals. Accordingly, the method of the invention is typically performed in a mammal which has already made an immune response to the immunogenic polypeptide and who may have, for example, circulating antibodies which react with, or immunocompetent memory cells specific for, the immunogenic polypeptide.

Typically, the relevant nucleic acid sequences are introduced into the cell in vitro. Numerous methods of introducing nucleic acid sequences into eukaryotic cells are known, including transfection, transduction, electroporation, cell fusion and the like. Any of these methods may be used in the present invention, and may generally be referred to as transformation.

Similarly, methods of introducing the transformed cell into the subject are well known. Conveniently this is done by infusion or injection into the subject's bloodstream. Typically between 10⁵ and 10⁸ cells (preferably between 10⁶ and 10⁷) will be introduced into the subject's bloodstream. It will be appreciated that introduction of foreign cells into a subject is likely to create an immune response against foreign antigens on the cell, so generally the cells will be tissue-matched with the recipient subject. Most conveniently, the cells will be autologous cells originally obtained from the subject (e.g. from peripheral blood, or from bone marrow), transformed in vitro with the relevant nucleic acid sequences, and then re-introduced into the subject. Typically, the in vitro stages of the method will generally comprise a selection process to select those cells successfully transformed, and/or a growth stage, to increase the numbers of transformed cells. Methods of in vitro selection and growth of cells are well known to those skilled in the art and form no part of the present invention.

Generally, where the method involves regulating the expression of a potentially immunogenic therapeutic polypeptide, it will be desirable to cause a delay in expression of the polypeptide after the cell is introduced into the subject, for reasons explained in greater detail below. Accordingly, the cell is generally transferred from conditions in vitro in which expression of the immunogenic polypeptide is fully repressed, to conditions in vivo in the subject in which the expression of the immunogenic polypeptide is no longer down-regulated. However, the invention is such that it takes a significant period which may be referred to as the "delay interval" (typically 2 to 10 days, preferably 4 days or longer) for the cell to move from the substantially fully-repressed state to the substantially fully-expressed state.

The cell may be in a state in which the immunogenic polypeptide is substantially fully repressed in vitro by exposure to appropriate (non-toxic) concentrations of the regulatory drug (e.g. tetracycline or an anaogue thereof), and when introduced into a regulatory drug- free subject eventually enters a state in which the immunogenic polypeptide is substantially fully expressed. Alternatively, as described above, because of the versatility of the TRS and the variants thereof and of other drug-regulatable promoter systems available to the person skilled in the art, the cell may be maintained in a fully-repressed state in vitro in the absence of regulatory drug, and then introduced into a subject receiving appropriate doses of the regulatory drug, so as to cause the immunogenic polypeptide to be fully expressed (after an appropriate delay).

Generally it is preferred that the presence of regulatory drag inhibits expression of the immunogenic polypeptide, as subsequent removal of the cell from regulatory drug exposure normally gives a longer lag phase or delay before induction of expression of the immunogenic polypeptide.

In most embodiments, the regulatory drug will be tolerated when administered to a subject. Accordingly, if desired, in those situations where the transformed cells are exposed in vitro to the regulatory drug so as to inhibit expression of the immunogenic polypeptide, the regulatory drag may also be administered to the patient before, during or after introduction of the transformed cells. This may be desirable for prolonging the delay interval, or for exposing the cells to intermediate concentrations of regulatory drug so as to regulate expression of the immunogenic substance or, if the patient should experience an adverse reaction to the expression of the immunogenic polypeptide, the regulatory drag could be administered in a sufficient dose to prevent further expression of the immunogenic polypeptide.

Methods of administration to the subject of the regulatory drag (if required) are well known to those skilled in the art and include oral administration (in any form), slow- release pellets and implantation of a diffusion pump, in addition to more conventional methods such as injection, infusion, inhalation or ingestion. Adjustment of the concentration of regulatory drag in the subject can be made by trial-and-error, by analysis of clinical symptoms or markers. As an example, if tetracycline or a tetracycline analogue is administered to a subject, the dosage is conveniently adjusted to achieve a serum concentration between about 0.05 and l.Oμg/ml.

The length of the delay interval can be altered by altering the concentration of regulatory drag to which the transformed cells are exposed (for example, if very high concentrations are used to pre-treat the cells, it will take longer for the intracellular concentration of drug to fall to a particular concentration once the cells are removed). Also, some drugs have a greater regulatory effect than others (e.g. doxycycline can have a greater effect on the TRS than tetracycline).

In a particular embodiment it is envisaged that the immunogenic polypeptide is one which exerts a therapeutic effect on a solid tumour in the subject. The subject is typically a human patient, but the method of the invention is potentially applicable to any mammalian subject, such as domesticated mammals, farm animals and so on. Thus the immunogenic polypeptide may be, for example, a cytotoxic agent (e.g. an "immunotoxin" - that is, a toxic moiety fused to an immunoglobulin binding domain, or other targeting moiety having a specific binding activity), or an agent such as an immunoglobulin, antibody, bispecific antibody or any of the other known variants of antibodies (e.g. scFv) to recruit tumour- infiltrating lymphocytes (TILs) into the tumour.

In one embodiment, the drug-regulatable promoter controls the transcription of a replicable viral genome or a viral vector (comprising the sequence encoding the immunogenic polypeptide). Suitable constructs for achieving this embodiment of the invention have been disclosed by, inter alia, Hofman et al, (1996) and Shockett et al, (1995) cited elsewhere. In preferred embodiments the replicable viral genome comprises substantially that of an adenovirus or a paramyxovirus (which genome may be artificially altered by conventional DNA manipulation techniques) . The person skilled in the art will appreciate that the genome need not contain (and conveniently does not contain) the entire genetic information of the virus, but contains sufficient to allow for the production of infectious viral particules when introduced into a susceptible host cell.

If the cells introduced into the subject (with expression of the immunogenic polypeptide repressed) are able to migrate through blood vessel walls (a process known as extravasation), they may be able to penetrate into a solid tumour. Examples of such cells with this capability are leukocytes. The efficiency of tumour attack by the introduced leukocytes may be enhanced by causing the cells to express targeting entities on their cell surface (see. for example, Eshar et al, 1993 Proc. Natl. Acad. Sci. USA 90, 720; Hwu et al, 1993 J. Exp. Med. 178, 361; Stancovski et al, 1993 J. Immunol. 151, 6577; and Brocker et al, 1996 Eur. J. Immunol. 26, 1770). Once within the tumour, the leukocytes can exert their therapeutic effect (e.g. cytotoxic action, or recruitment of macrophages and lymphocytes).

However, targeting of the introduced leukocytes to the tumour takes some time. Thus, expression of the immunogenic polypeptide during this time is preferably avoided as it may cause: (a) collateral damage to non-malignant cells and/or (b) interaction of the immunogenic protein with components of the subject's immune system (especially circulating antibodies). This latter point is particularly pertinent if repeated administrations of the leukocytes is required, as this would cause efficient induction of immune responses to the immunogemc polypeptide, which would tend to interact with the administered leukocytes and prevent them from reaching the target tumour. These problems can be overcome by the method of the present invention, in which the therapeutic (immunogenic) polypeptide is only allowed to become expressed at high levels after a significant time delay, by which point the administered leukocytes will have penetrated the target tumour, thereby preventing interception by the immune system and minimising collateral damage to non-malignant cells. Accordingly, the method of the invention can be performed in a subject which has already made an immune response to the immunogemc polypeptide and who may have, for example, circulating antibodies which react with, or immunocompetent memory cells specific for, the immunogenic polypeptide.

Any solid tumour, accessible by blood-borne cells, could be available for treatment by the method of the invention. Conveniently the cell introduced into the subject also comprises a cell-surface component which targets the leukocyte to a marker expressed on the surface of the tumour cells. These may be, for example, chimeric "T body" targeting components, known to those skilled in the art (see disclosures of Eshar et al, Hwu et al, Stancovski et al, and Brocker et al, cited above).

It may be desirable to modify the components of the drug-regulatable promoter system employed in the method so as to reduce their immunogenicity (e.g. by modifying any DNA-binding protein so as to remove certain epitopes). Conveniently, if present, the DNA-binding protein will comprise a nuclear localization signal (NLS), so as to minimise the amount which might become presented to the subject's immune system. Other techniques useful in this regard are disclosed in WO 96/01313.

In a second aspect, the invention provides a cell transformed with a nucleic acid sequence encoding a polypeptide which is immunogenic to a normal human subject, the nucleic acid sequence being operably linked to a drug-regulatable promoter, such that expression of the immunogenic polypeptide by the cell may be controlled by altering the concentration of regulatory drug to which the cell is exposed. The transformed cell is typically for use in the method defined above.

In a third aspect the invention also provides a composition for use in a gene therapy method, comprising a plurality of cells in accordance with the second aspect as defined above, in a physiologically acceptable diluent medium (e.g. saline or phosphate-buffered saline, tissue culture medium etc.).

The invention further provides, in a fourth aspect, a method of making a composition for use in gene therapy, the method comprising: (a) obtaining a sample of cells from a mammalian subject; (b) transforming the cells with a nucleic acid sequence encoding a heterologous immunogenic polypeptide, said nucleic acid coding sequence being operably - linked to a drug-regulatable promoter; and (c) mixing the cells with a physiologically acceptable diluent. The method defined above conveniently comprises additional steps, such as, after performing step (b) screening and selecting for those cells successfully transformed and, optionally, growing the selected cells in vitro to expand the numbers of transformed cells. Screening and selection may conveniently be performed by a fluorescence-activated cell sorter. Desirably, performance of the method of the fourth aspect defined above will result in the production of a composition suitable for use in the method of the first aspect of the invention.

The invention will now be further described by way of illustrative example and with reference to the accompanying drawings, in which: Figures 1A and IB are schematic representations of nucleic acid constructs referred to in the example below;

Figures 2A, 2B and 3 show representative FACS data;

Figure 4 is a graph showing expression of a chimeric polypeptide (as a percentage of expression in control cells) against time; and

Figures 5A and 5B are bar charts showing the levels of IL-2 production (in picograms/ml) by T lymphocytes exposed to various concentrations of tetracycline or the tetracycline analogue, doxycycline.

Detailed Description of an Embodiment of the Invention

Example 1

The inventors have evaluated the utility of the tetracycline-controlled transactivator system as a means to temporally regulate the expression of a surface molecule in a human T cell line. Using a vector containing both the transactivator and the expression gene unit, we were able to generate stably transfected Jurkat T cell lines in which the expression of a chimeric TCR (chTCR) molecule could be efficiently regulated. Depending on the tetracycline analogue used and its concentration, the induction of chTCR can be reversibly repressed to a greater or lesser extent. Futhermore, we have shown that fully repressed T cells can not be activated to produce IL-2 via this chimeric receptor, indicating that reversible functional inactivation of redirected T cells is possible.

The time-course to repress gene expression to basal levels was significantly shorter than the time-course for gene expression to reach maximal levels after drag removal, and the delay in resumption of promoter activity varied considerably depending on the concentration of doxycycline (a tetracycline analogue) used for repression. The relevance of these data to current ideas on T cell mediated immunotherapy is discussed. Materials and Methods

Reagents. The mAbs used included SPvT3b (mouse IgG2a) (10) and YTH913.12 (rat IgG2b) (Serotec Ltd., Oxford, UK) specific for human CD3e and CD28 molecules respectively. For direct staining the following FITC-conjugated antibodies were used: UCHT-1 (anti-CD3e, a mouse IgGl Serotec; goat polyclonal antisera to mouse λ-light chain (Southern Biotechnology Associates, Inc, Birmingham AL); and goat polyclonal antisera to mouse IgG (γ-chain specific) (Sigma Chemical Co. , St. Louis, MO). Bovine serum albumin (BSA) was conjugated with 4-hydroxy-5-iodo-3-nitrophenyl acetyl (NIP) (Cambridge Research Biochemicals, Northwich, UK) in a molar ratio of 10: 1 (NIP₁₀-BSA) (11). Tetracycline hydrochloride (Sigma) was dissolved at a concentration of 0.5 mg/ml in culture medium. Doxycycline hydrochloride ("Dox") (Sigma) was dissolved in 0.02N HC1 at a concentration of 1 mg/ml and further diluted in culture medium. The antibiotic solutions were freshly prepared on the day of use and diluted to the appropriate concentrations.

Vector Construction. Plasmids pUHD 15-1, containing the tTA transactivator gene transcribed from the human CMV immediate early (CMV IE) promoter/enhancer, and pUHD 10-3, containing a tTA-responsive promoter (TRP, heptamerized tetO sequences (TetO)7 fused to a human CMV immediate early minimal promoter [PhCMV*- 1]), were kindly provided by H. Bujard (8). Plasmid pCS was constructed by removing a 1308 bp Sal I fragment, containing the CMV IE promoter, the multiple cloning site and the SV40 polyadenylation signal, from pCEP4 backbone (Invitrogen, San Diego, CA). A 6723 bp Nru I-Cla I digested fragment from pCS was blunt ended with Klenow (Cambio, Cambridge, UK) and inserted into the Xho I site of plasmid pUHD 15-1 following the treatment of this site with Klenow and calf intestinal alkaline phosphatase (CIP, Boehringer Mannheim GmbH, Germany). The resulting plasmid containing both the tTA and the hygromycin transcription units in opposite directions was designated pCRAZY.

A chimeric NIP-specific scFv-TCR f molecule was constructed as described previously (11) and cloned into the plasmid pUHD 10-3. To do this the Hind III site from pUHD 10-3 was removed by cleavage with Hz>zd III followed by Klenow fill-in and blunt-end ligation resulting in pLAV5. To construct pLAV6, a 1342 bp EcoR l-Xba I fragment derived from the plasmid pVACl.aNIP.TCR ζ (described in reference 11), containing a human VH1 leader sequence and a chimeric NlP-specific TCR ξ molecule, was cloned into the EcoR I-Xba I poly linker site of pLAV5. The 91 bp EcoR I-H d III fragment containing a Rous Sarcoma virus (RSV) promoter partial sequence was removed from pLAV6 by digestion with EcoRl and Hind III, Klenow fill-in and blunt end ligation to produce plasmid pLAV7. A polylmker containing restriction sites unique to the vector c o n s t r u c t , H i n d l l l - B g I I I - E c o R W - C l a I , ( 5 8 2 6 : 5'-CATCGATCGAACTGATATCAGCAGATCTCAGAAGCTTAAT-3' Seq. ID No. 1) and 5827: 5'-ATTAAGCTTCTGAGATCTGCTGATATCAGTTCGATCGATGACGT-3' Seq. ID No. 2) was ligated into the Ssp I-. αtII site of pLAV7 resulting in pLAV8. To construct a single plasmid with both the tTA transactivator gene and the chimeric NIP-specific scFv-TCR f gene under the control of TRP in antisense orientation (relative to the tTA transcription unit) the plasmid pCRAZY was digested with Xmn I and a Bg\ II linker (New England Biolabs, Inc. , Beverly, MA) was introduced at this site. After digestion with Bg\ U and Hind III a 9386 bp fragment was inserted into the Bgl Ϊ -Hind III site of pLAV8. The resulting plasmid was designated pLAV12 (Fig. 1A).

Cell Culture and Transfections. The Jurkat T cell line (clone E6-1) was maintained in RPMI 1640 supplemented with 10% foetal calf serum (FCS), 2 mM L-glutamine, 100 U/ml penicillin, 100 μg/ml streptomycin and 25 mM ΗEPES buffer (all from GIBCO-BRL, Gaitersburg, MD), referred to as complete medium (CM). To generate stable cell lines Jurkat cells were transfected with linearized plasmid DNA (10 μg) by electroporation, at 250 mV and 960 μF, as described previously (12). Transfectants were selected in CM supplemented with 0.4 mg/ml of hygromycin B (Calbiochem, San Diego, CA). Stable cell lines were established after 3-4 weeks and analyzed by FACS for expression of the chTCR. To select a population of chTCR ξ expressing cells, stable Jurkat cells transfected with the linearized pLAV12A or pCEP4.aNIP.TCR ξ derived plasmid fragments were FACS sorted (see below). The resultant populations were cloned twice by limiting dilution and screened for protein expression by flow cytometry.

Flow Cytometry and Cell sorting. Expression of cell surface proteins was performed by standard direct immunofluorescence as described (13) using saturating amounts of FITC -conjugated antibodies. Dead cells were excluded from analysis using a combination of propidium iodide and forward light scatter. Appropriate FITC isotype-matched irrelevant Abs were used in all experiments. The samples were analysed with a FACScan^® machine (Becton Dickinson, Mountain View, CA). A minimum of 20,000 cells was analysed for each sample. Subsequent re-analysis of data was performed using the CELLQuest software (version 1.2) (Becton Dickinson). Additionally, cells stained with FITC-conjugated goat antisera to mouse λ light chain were sorted under sterile conditions on a cell sorter (FACScalibur, Becton Dickinson).

IL-2 Release Assay. The cells were pre-incubated at a concentration of 5xl0⁵/ml for 48 hours in CM in the absence or presence of tetracycline or doxycycline at the indicated concentrations. Subsequently the cells were washed, counted and stimulated (10⁵/well) in triplicate with plastic-immobilised NIP10-BSA conjugates (iNIPlO-BSA) or plastic- immobilised anti-CD3e mAb (ianti-CD3) in fresh CM alone or in the presence of the drugs (11). The plates were incubated at 37°C in 5% C0₂. After 20 hours supernatants were harvested and assayed for IL-2 activity using an ELISA kit (Genzyme Diagnostics, Cambridge, MA).

Results

Design of the tet-regulatable Vector.

To determine if the expression of a chimeric, potentially immunogenic, TCR could be pharmacologically regulated in T cells, the inventors constructed the plasmids pLAV12 and pCEP4.aNIP.TCR " (Figure 1). Figures 1A and IB are schematic maps of representative plasmid fragments used in the experiments showing the predicted structure after integration into the host genome: (A) pLAV12 and (B) pCEP4.aNIP.TCRf, respectively. The direction of transcription is indicated by arrows.

Both constructs encode a chimeric (ch) TCR molecule that has been described previously (11), which comprises the antigen combining site of the hapten-specific (NIP) mAb B1.8 (14) fused to the transmembrane and cytoplasmic regions of the human TCR f chain (15). pLAV12 (Fig. 1A) is a tetracycline-regulatable construct containing all the components of the TRS with the tTA gene under the control of the constitutive CMV IE promoter and the gene coding for the chTCR inserted under the control of the tTA-responsive promoter. In an attempt to reduce potential cis-regulatory enhancement of TRP activity, due to proximity to the other enhancer and promoter elements present on the vector, the tTA-response cassette was separated from the other transcriptional units by a 2500 bp fragment containing the pUC derived ColEl origin of replication and the 3-lactamase gene (Fig. 1 A). In the control construct, pCEP4.aNIP.TCRf. the chTCR molecule was under the control of the constitutive CMV IE promoter (Fig. IB).

Both constructs encode a hygromycin resistance marker gene transcribed by a constitutive promoter. To promote the stable integration of these DNA constructs in the designed configuration in transfected T cells, a linearized Avr l-Sap I 9975 bp DNA fragment (Fig. 1A) from plasmid pLAV12 and a linearized Avr H-EcoRV 7551 bp fragment (Fig. IB) derived from plasmid pCΕP4.aNIP.TCRf, both lacking the EBV replication origin .and the EBNA-1 gene (Fig. 1), were used to transfect Jurkat cells.

tTA-dependent Expression of the Chimeric scFv Gene Construct.

Jurkat E6-1 cells were transfected with linearized fragments of pLAV12 and pCEP4.aNIP.TCRi". To select for higher expression of the chTCR. the stably transfected hygromycin-resistant cells were FACS sorted after staining with a FITC-labelled goat anti mouse λ-light chain antiserum. Most of the isolated pLAV12 transfectants (JLAV12S) and pCEP4.aNIP.TCRf transfectants (JN3S cells), expressed the chTCR although there was considerable heterogeneity in the absolute levels of expression (Fig. 2A, 2B).

Figure 2 shows representative results confirming the regulation of the chTCR gene expression by tetracycline analogues. In Figure 2A, stable transfected uncloned JLAV12S (left hand side) and JN3S Jurkat (right hand side) cell populations were cultured for 48 hours in tetracycline-free medium (CM, upper row of panels) or in the presence of 1 μg/ml of Tet (broken line) or Dox (solid line)(lower row of panels) and the surface expression of chTCRs was examined after staining with FITC-conjugated goat antisera to mouse λ light chain. Figure 2B comprises four panels showing a timecourse of inactivation of chTCR gene expression in JLAV12S cells zero hours (top left), 8 hours (top right), 12 hours (bottom left) or 24 hours (bottom right) after addition of Dox at 1 μg/ml. In both Figures 2A and 2B negative controls (FITC-conjugated goat antisera to mouse IgG) are overlaid (filled curve). The fluorescence channel number is plotted along the x axis, and the y axis represents the relative cell number.

The selected population of pLAV12 transfectants (JLAV12S) was then cloned by limiting dilution and two subclones expressing the chTCR at low (2E11) and intermediate (1F5) levels (Figure 3) were selected for further study.

Regulation of the Chimeric scFv-TCRζ Gene Expression.

To determine whether the expression of the chTCR could be suppressed by tetracyclines, JLAV12S and JN3S cells (5xl0⁵/ml) were incubated for 48 hours in the presence of 1 μg/ml of tetracycline (Tet) or its analogue doxycycline (Dox). At this concentration most of surface chTCRs (90%) were down-regulated in JLAV12S cells, but were not affected in JN3S cells (Figure 2A). Also, surface staining with anti-CD3e mAbs demonstrated that the amount of TCR CD3 complex remained constant in both cell populations (not shown). No changes in cell viability were observed at this concentration, assayed using trypan blue staining (data not shown).

To study the time-course of inactivation of gene expression, JLAV12S cells were analysed at different times after addition of the antibiotics at 1 μg/ml (Fig. 2B). A slight reduction was observed within 8 hours of exposure to the drugs and maximum repression was achieved within 24 hours, when the expression of the chTCR fell to less than 10 percent of the level observed in the absence of tetracyclines at the same time point (Figure 2B). Similar results were observed in the 1E5 and 2E11 clones where the level of chTCR was reduced to about 10% (1E5) and 20% (2E11) of its maximal expression (Figure 3). The time-course of gene repression was very similar in response to both antibiotics (Tet or Dox) in all analysed populations. It is important to note that the percentage of cells in which tetracyclines did not down-regulate the expression of chTCRs was < 0.5%, not affecting the overall regulation in the whole population of JLAV12S cells (Figure 2A). A dose-response curve of gene repression was determined using different concentrations of Tet or Dox. After 48 hours of treatment, the cells were harvested and the expression of the chTCR was studied by FACS analysis. Typical results are shown in Figure 3.

Stably transfected uncloned (JLAV12S, left hand column) and cloned (1F5, middle column; and 2E11, right hand column) Jurkat cell populations were cultured for 48 hours in the presence of different concentrations (Ong/ml top row, O.lng/ml second row, lng/ml third row, and 1 Ong/ml bottom row) of Tet (broken line) or Dox (solid line) and the surface expression of scFv-TCR f molecules was examined. Negative controls (FITC-conjugated goat antisera to mouse IgG) are overlaid (filled curve). The fluorescence channel number is plotted along the x axis, and the y axis represents the relative cell number.

Referring to Figure 3, in both clonal populations (1E5 and 2E11) the expression of chTCRs was maximally repressed at 100 pg/ml (O.lng/ml) of Dox, with no further increment in the level of repression at higher concentrations. Partial activity of the TRP was observed in the concentration range of 1 pg/ml to 100 pg/ml of Dox (data not shown). In JLAV12S cells maximal repression was observed at Dox concentrations greater than or equ.al to 1 ng/ml. For tetracycline maximal repression occurred, in all the analysed cell lines, at concentrations greater than or equal to 10 ng/ml. The induction of the chTCR gene was only partially repressed at tetracycline concentrations of 100 pg/ml to 1 ng/ml.

To study the kinetics of recovery of TRP-driven gene expression after withdrawal of tetracyclines, stable transfected uncloned JLAV12S cells were cultured for 48 hours in the presence of different concentrations of Dox (1 ng/ml to 1 μg/ml). After three washes the cells were incubated (5xl0⁵/ml) in tetracycline-free CM in new plates and the surface expression of chTCRs was examined every 24-48 hours after staining with FITC-conjugated goat antisera to mouse λ light chain. The results are shown in Figure 4. Similar experiments were also performed with 1F5 cells (data not shown).

100% values correspond to the median fluorescence of control untreated cells after staining with FITC-conjugated goat antisera to mouse light chain. The values are the percentage of chimeric TCRf molecules expressed on tetracycline treated cells from each cell population as compared with the amount of chimeric TCRf molecules on control untreated cells (as 100%). Referring to Figure 4, after treating the cells with 1 μg/ml of Dox (filled circles), recovery of chTCR expression was not apparent at 192 hours after removal of the drug and was first detected on the cell surface after 216 hours, with full recovery of expression after 288 hours. In contrast, the TRP remained repressed for only 24 hours after drug removal, when cells were pretreated with 1 μg/ml Tet, with maximal levels of chTCR expression being reached after 72 hours (data not shown). Treating the cells with lower concentrations of Tet (not shown) or Dox (1 ng/ml - open squares, 1 Ong/ml - filled squares, lOOng/ml - open circles) resulted in earlier recovery of the activity of the TRP.

Example 2

Functional Study

It has previously been shown that the chTCR employed in this study is able to mediate specific recognition of its cognate antigen (NIP conjugated to BSA), soluble or plastic immobilized, resulting in the production of IL-2 by the transfected T cells (11). In the current study the inventors found consistently that stimulation of chTCR expressing cells (JN3S, JLAV12S, 1F5 and 2E11) with plastic immobilized NIP₁₀-BSA conjugates induced IL-2 secretion (data not shown). The level of IL-2 production varied between different transfectant cell populations, but in general it was similar to that observed in the same cell population in response to standardised stimulation with anti-CD3e mAb immobilized in micro titre wells (not shown).

Given that high concentrations of tetracycline have been shown to interfere with the process of T cell activation (16), the inventors studied the effects of increasing concentrations of Tet and Dox on the anti-CD3e induced IL-2 secretion of Jurkat cells. IL-2 secretion was unaffected by 100 ng/ml or lower concentrations of doxycycline but was inhibited by 25%> at a doxycycline concentration of 1 μg/ml (not shown). Tetracycline at concentrations lower than or equal to 1 μg/ml did not influence the anti-CD3e induced IL-2 secretion, which was similar to that observed in untreated cells (not shown). These results indicate that is possible to induce maximal TRP repression at concentrations of doxycycline more than 1000-fold lower than the threshold at which immunomodulating effects on human T cells are first manifest.

The inventors next determined whether tetracycline-mediated suppression of the chTCR could induce a reversible state of antigen unresponsiveness in the transfected Jurkat cells. JLAV12S and JN3S cells were preincubated for 48 hours in increasing concentrations of Dox and Tet and then stimulated with immobilized NIP-BSA, following which their IL-2 production was measured. In situations of full TRP repression JLAV12S cells down-regulated 90%> of surface chTCRs ( see Figure 3), and were completely unresponsive to stimulation with iNIPlO-BSA conjugates (see Figure 5A).

Figures 5A and 5B are bar charts showing IL-2 production (in pg/ml) by transfected JLAV12S (5A) or JN3S (5B) cells stimulated with iNIPlO-BSA in the absence or presence of Tet or Dox. Cells were preincubated for a 48 hours period in the absence or presence of the drags (at the indicated concentration in ng/ml), washed and stimulated (lOVwell) with plastic immobilised NIP10-BSA conjugates (50 μg/ml) in fresh CM alone (solid bar) or in the presence of Tet (shaded bars) or Dox (open bars) at the indicated concentrations. One of two similar experiments is shown.

Down-regulation of about 75% of chTCRs was associated with low IL-2 production, whereas no inhibitory effect was observed when the level of chTCR expression was about 50%) of that observed in absence of tetracycline (Figure 5A). These results indicate that the number of surface chTCR molecules expressed by JLAV12S cells in situations of full TRP repression is not enough to reach the activation threshold required for optimal T cell function. In contrast, when JN3S cells were stimulated with iNIPlO-BSA conjugates, there was no inhibitory effect of tetracycline and of doxycycline at a concentration of less than 1 μg/ml (Figure 5B).

Discussion

The inventors have used a single vector containing all of the components of the TRS for the pharmacological regulation of a foreign gene expressed in a human T cell line. The TRS comprises the tTA gene, usually driven by a constitutive promoter, and a gene of interest immediately downstream of the tTA-responsive promoter (8). To facilitate the application of the TRS to T cells, the inventors constructed a self-contained plasmid vector encoding both components of the TRS, as well as a hygromycin selectable marker gene under the control of a constitutive promoter. The new vector overcomes the efficiency losses inherent in co-transfection with the original two-plasmid based TRS system (8) and ensures the integration of equal copy numbers of the tTA and reporter gene units in a direct cis-configuration at the same chromosome locus.

Using this stable expression system as a model for integrated gene therapy approaches the inventors have demonstrated that a scFv-TCRf chimeric molecule can be functionally expressed in a human T cell line, and that its expression can be pharmacologically down-modulated. In the absence of tetracyclines the level of expression of the chTCR was comparable to that observed when the chimeric gene was driven by the strong CMV IE enhancer/promoter. Efficient tetracycline-dependent suppression of gene expression was observed in all the studied T cell transfectants. Depending on the dose and analogue employed, the expression of chTCRs could be repressed to a greater or lesser extent. This indicates that the TRS is applicable to T cells, achieving function^ levels of tr.ansactivator expression without any evident toxic squelching effects of the VP16 domain.

Variable potencies have been demonstrated for different tetracycline analogues in previous studies on the TRS with doxycycline exhibiting 100-fold greater potency than tetracycline (17). In our system maximum repression occurred at a concentration of 10 ng/ml tetracycline whereas doxycycline caused full repression of the TRP over the range of 100 pg/ml to 1 ng/ml.

The time-course for the decline in chTCR expression upon exposure to Tet or Dox was short and suggests that, like wild-type TCRf chains the chimeric TCRf chains exhibit rapid turnover (18). In this regard it is interesting to note that the turnover of f chains in Jurkat cells is similar to that in primary T cells (18). In contrast to the rapid suppression of gene expression, its recovery after removal of doxycycline was much slower. The delay before the commencement of recovery of chTCR gene expression varied in direct proportion to the concentration of doxycycline that was used for repression. This sustained repression and relatively slow recovery of gene expression which occurs upon removal of tetracycline analogues has been observed in other cell types and could be of interest in the development of new T cell immunotherapy approaches and new immune evasion strategies.

Expression of the chimeric TCR conferred responsiveness to NIP-BSA conjugates, as evidenced by IL-2 secretion upon exposure to the antigen. Doxycycline was shown to completely abrogate this antigen responsiveness at concentrations that had no other immunomodulating effects on human T cells, and well below the tissue concentrations that are achieved clinically when doxycycline is used to treat infection (19). Thus by treating engineered T cells with doxycycline before their administration, it should be possible to switch off expression of the transgene for a predetermined period of time, thereby limiting collateral damage to normal tissues that may express low levels of the t.argeted antigen.

Complete suppression of transgene expression was not fully achieved in the current study. The characteristics of the employed vector and/or cell type-specific factors may account for the basal level of transcription observed in the uninduced state (21, 22). In addition, the potential immunogenicity of the tTA protein must be t.aken into consideration. In the vector that we have employed the transactivator is driven by a constitutive promoter ensuring consistent expression independently of the presence of tetracycline, and may therefore possibly elicit an immune response against the genetically modified T cells, even in the repressed state. However, this is not the case for a newer generation of enhancerless tetracycline-responsive vectors, in which tetracycline prevents the tTA protein from binding to the TRP, thereby suppressing its own expression as well as the expression of the reporter gene by way of an autoregulatory circuit (17, 22). This arrangement gives enhanced suppression of transgene expression (to negligible levels in preliminary experiments) and ensures that the abundance of the tTA protein is greatly reduced in the fully repressed state (22). References

All publications cited in the specification are incorporated herein by reference.

1. Mule et al, 1984 Science 225: 1487.

2. Rosenberg et al, 1986 Science 233:1318.

3. Eshhar et al, 1993 Proc. Natl. Acad. Sci. USA. 90: 720.

4. Hwu et al, 1993 J. Exp. Med. 178: 361.

5. Stancovski et al 1993 J. Immunol. 151 : 6577.

6. Brocker et al 1996 Eur. J. Immunol. 26:1770.

7. Yarranton 1992 Curr. Opin. Biotechnol. 3:506.

8. Gossen & Bujard 1992 Proc. Natl. Acad. Sci. USA. 89:5547.

9. Gossen et al 1993 Trends Biol. Sci. 18:471.

10. Spits et al 1985 Eur. J. Immunol. 15:88.

11. Alvarez- Vallina & Hawkins 1996 Eur. J. Immunol. 26: 2304.

12. Patten et al 1992 J. Immunol. 150:2281.

13. Alvarez-Vallina et al 1993 J. Immunol.150: 8.

14. Hawkins et al 1992 J. Mol. Biol. 226: 889.

15. Weissman et al 1988 Proc. Natl. Acad. Sci. USA. 85: 9709.

16. Kloppenburg et al 1995 Clin. Exp. Immunol. 102:635-641.

17. Hofmann et al 1996 Proc. Natl. Acad. Sci. USA. 93: 5185-5190.

18. Ono et al 1995 Immunity 2:639-644.

19. Houin et al 1983 Br. J. Clin. Pharmac. 16:245.

20. Shockett & Schatz 1996 Proc. Natl. Acad. Sci. USA. 93:5173.

21. Ackland-Berlund & Leib 1995 BioTechniques. 18:196.

22. Shockett et al 1995 Proc. Natl. Acad. Sci. USA. 92:6522.

Claims

1. A method of regulating the expression in a human or animal subject of a nucleic acid sequence encoding a polypeptide which is immunogenic in the subject; the method comprising introducing into the subject a cell comprising the nucleic acid sequence encoding the immunogenic polypeptide, said sequence being operably linked to a drug- regulatable promoter; and altering the concentration of regulatory drug to which the cell is exposed.

2. A method according to claim 1, in which the cell is a leukocyte.

3. A method according to claim 1 or 2, wherein the cell is a B lymphocyte, T lymphocyte, monocyte or macrophage.

4. A method according to any one of claims 1, 2 or 3, wherein the subject has made an immune response to the immunogenic polypeptide.

5. A method according to any one of the preceding cl ms, wherein the subject has circulating antibodies which react with the immunogenic polypeptide.

6. A method according to any one of the preceding claims, wherein the subject has immunocompetent memory cells which are specific for the immunogenic polypeptide.

7. A method according to any one of the preceding claims, wherein prior to introduction of the cell into the subject the expression of the immunogenic polypeptide is substantially inhibited in vitro, and wherein expression of the immunogenic polypeptide reaches a maximum level in the subject after a delay interval.

8. A method according to claim 7, wherein expression of the immunogenic polypeptide is inhibited in vitro by exposure of the cell to the regulatory drug, and wherein expression in the subject is induced after a delay interval, the subject being substantially free of the regulatory drug.

9. A method according to claim 7, wherein expression of the immunogenic polypeptide is inhibited in vitro by substantial absence of the regulatory drag and wherein expression in the subject is induced after a delay interval by administration to the subject of the regulatory drug.

10. A method according to any one of the preceding claims, wherein the regulatory drug is selected from the group consisting of: tetracycline or an analogue thereof (as herein defined); glucocorticoid steroids; sex hormone steroids; lipopolysacchari.de (LPS); and isopropylthiogalactoside (IPTG) .

11. A method according to any one of the preceding claims, wherein the immunogemc polypeptide exerts a therapeutic effect in the subject.

12. A method according to any one of the preceding claims, wherein the immunogenic polypeptide exerts an anti-tumour effect in the subject.

13. A method according to any one of the preceding claims, wherein the nucleic acid sequence encodes a replicable viral genome or a viral vector.

14. A cell transformed with a nucleic acid sequence encoding a polypeptide which is immunogenic to a normal human subject, the nucleic acid sequence being operably linked to a drug-regulatable promoter, such that expression of the immunogenic polypeptide by the cell may be controlled by altering the concentration of regulatory drug to which the cell is exposed.

15. A leukocyte according to claim 14.

16. A cell according to claim 14 or 15, for use in the method of any one of claims 1-13.

17. A composition for use in the method of any one of claims 1-13, comprising a plurality of cells in accordance with any one of claims 14, 15 or 16 and a physiologically acceptable diluent.

18. Use of a cell according to claim 14, in the preparation of a composition used to perform the method of any one of claims 1-13.

19. A method of making a composition for use in gene therapy, the method comprising: obtaining a sample of cells from a mammalian subject; transforming the cells with a nucleic acid sequence encoding a heterologous immunogenic polypeptide, said nucleic acid coding sequence being operably linked to a drug-regulatable promoter; selecting those cells successfully transformed; and mixing the selected cells with a physiologically acceptable diluent.

20. A method according to claim 19, wherein performance of the method produces a composition suitable for use in the method of any one of claims 1-13.

21. A method of regulating the expression of a nucleic acid sequence encoding a heterologous polypeptide in a leukocyte, comprising introducing into the leukocyte the nucleic acid coding sequence operably-linked to a tetracycline-operator sequence, and a sequence encoding a tetracycline-sensitive DNA-binding expression-regulating polypeptide; and altering the concentration of tetracycline (or analogues thereof) to which the leukocyte is exposed, so as to regulate expression of the coding sequence.