REVERSIBLE MHC MULTIMER STAINING FOR FUNCTIONAL PURIFICATION OF ANTIGEN-SPECIFIC T CELLS

Abstract: The present invention relates to a new method for reversible staining and functional isolation or characterization of cells, e.g. antigen-specific T cells. With this technique, the original functional status of cells can be substantially maintained after their identification and purification. Thus, this new method is of broad benefit for basic research and clinical applications.

Description: Reversible MHC multimer staining for functional purification of antigen-specific T cells 5 Description The present invention relates to a new method for reversible staining and functional isolation or characterization of cells, particularly of antigen- specific T cells. With this technique, the original functional status of T cells o can be substantially maintained after their identification and purification. Thus, this new method is of broad benefit for basic research and clinical applications. Identification and purification of antigen-specific T cells without altering 5 their functional status is of great scientific and clinical interest. Methods for direct identification of T cells based on their antigen-specificity (ELISPOT assay (1 ), intracellular cytokine staining (2), secretion assay [affinity matrix] (3), MHC multimers (2, 4, 5)) have recently been developed. However, most of these identification techniques require in o vitro stimulation of T cells, which significantly changes the phenotype and functional status of the cells. To date, only the MHC multimer technology allows the identification and purification of antigen-specific T cells independent of their phenotype, but unfortunately, also conventional MHC multimer staining interferes with functional T cell analysis. 5 Yee et al. (J. Immunol. 162 (1999), 2227-2234) describe the isolation of high avidity melanoma-reactive cytotoxic T lymphocytes (CTL) from heterogeneous populations using peptide-MHC tetramers. The MHC tetramers are biotinylated and irreversibly conjugated to avidin containing o a fluorescent labeling group. A removal of the fluorescent marker under physiological conditions is thus not possible. Youde et al. (Cancer Res. 60 (2000), 365-371 ) describe the use of fluorescence labelled MHC tetramers to isolate human CTLs recognizing endogeneous human papilloma virus antigens. Also here biotinylated MHC molecules are used, which are irreversibly bound to streptavidin under physiological conditions so that a removal of the detection reagent from the T cells is not possible. US-A-5,985,658 describes a method for separating target cells from a plurality of cells which is based on a reversible high affinity interaction. The method comprises forming a target cell/cell binding reagent/first molecule/second molecule/solid support complex, wherein the cell binding reagent is specific for target cells, e.g. an antibody, wherein the first molecule reversibly binds to the second molecule, wherein one of the first and second molecules is calmodulin, and wherein the other of the first and second molecules is a calmodulin-binding peptide. After removal of non- target cells not attached to the solid support the binding between first and second molecule is reversed, thereby releasing target cells as separate cells from the plurality of starting cells. The disadvantage of this method is that the target cell binding agent must have a high affinity for the target cell, in order to enable an isolation of the target cell population. Thus, after removal of the binding between first and second molecule the target cell-specific reagent remains bound to the target cell. Werther et al. (J. Immunol. Meth. 238 (2000), 133-141 ) describe an antibody-based protocol for the isolation of carcinoma cells from mononuclear cell suspensions. The target cells are stained with an antibody linked to a magnetic bead and thus separated from the starting cell population. The magnetic bead is bound to the target cell via a DNA linker which may be digested with DNase in order to allow a further phenotypical characterization of the target cells. Also this method has the disadvantage that a high affinity target ceil binding reagent, i.e. an antibody has to be used, which cannot be easily removed from the target cell. Marelli-Berg et al. (J. Immunol. 244 (2000), 205-215) describe a protocol for the isolation of endothelial cells from murine tissue. Endothelial cells are labelled by specific antibodies and then purified via binding to microbeads. Also this method includes the above mentioned disadavantages. We here describe the development of a new method for identification and purification of antigen-specific T cell populations without substantially altering their functional status comprising a reversible staining procedure. More particularly, the reagents used for purifying a target cell population may be completely removed from the target cells. This approach combines the specificity and sensitivity of MHC multimer staining with preserving the functional status of the cells. T cells recognize processed antigen-fragments (epitopes) in the context of major histocompatibility complexes (MHC) (6). The specificity of T cells is determined by the structure of their T cell receptor (TCR), which is expressed at high levels on the cell surface. Early attempts to use the natural ligand of the TCR - the MHC/epitope complex - for direct identification of epitope-specific T cells were unsuccessful. The failure of this approach can largely be explained by the low affinity of TCR- MHC/peptide interactions, which are especially characterized by a high dissociation rate (Fig. 1A). Multimerization of MHC/peptide (e.g. to tetrameric molecules) increases the relative avidity of the interaction with TCRs to an extent which allows stable and epitope-specific binding on the T cell surface (Fig. 1 B). MHC multimer reagents conjugated with fluorochromes can be used for direct identification of antigen-specific T cells by flow cytometry. This method has revolutionized T cell research over the last few years, allowing to visualize antigen-specific T cell populations for the first time directly ex vivo, in animal models as well as in humans (7, 8). As long as MHC multimer staining is performed at 4°C, T cells can be identified and purified (FACS, MACS) without altering their original phenotype. However, since MHC multimer reagents represent the natural ligand bound to the TCR, placement of cells into in vitro cell culture or adoptive in vivo transfer into recipients (situations in which the temperature is shifted to 37°C) results in dramatic alteration of purified T cell populations (Figs. 3 + 4), including TCR intemalization, activation, overstimulation, and cell death. Thus, conventional MHC multimer technology allows direct visualization and phenotypic analysis of epitope-specific T cells. However, subsequent functional analysis and in vivo transfer of MHC multimer-stained and purified T cells is complicated by the persistence of TCR-MHC interactions and subsequently induced signaling events. Therefore, the development of methods to remove surface-bound MHC multimers after T cell staining and purification will be of great interest to the field of T cell immunology and is essential for the transfer of MHC multimer technologies to clinical applications and functional diagnostics. Thus, a subject matter of the present invention is a method for reversible staining of cells comprising the steps: (a) providing a sample comprising a cell having a receptor molecule, (b) contacting said cell with (i) at least one component specifically binding to said receptor wherein said at least one component is conjugated to at least one first partner of a binding complex, wherein the component and the receptor exhibit a low-affinity interaction, (ii) at least one further partner of said binding complex having at least two binding sites for said first partner and (iii) a detectable label bound to or capable of binding to (i) and/or (ii), wherein the first partner(s) and the further partner(s) of the binding complex are capable of forming a reversible bond and wherein an aggregate comprising at least two components (i), at least one component (ii) and at least one component (iii) is bound via the receptor molecule to said cell, wherein said cell is stained, (c) optionally separating said stained cell from other components of said sample and (d) optionally removing said staining from said cell by disrupting the reversible bond. A particularly preferred embodiment of the present invention is a method for reversible staining of T cells comprising the steps: (a) providing a sample comprising a cell having a T cell receptor (TCR) molecule, (b) contacting said cell with (i) at least one TCR binding ligand, e.g. a TCR binding peptide and an MHC molecule conjugated to at least one first partner of a binding complex, (ii) at least one further partner of a binding complex having at least two binding sites for the first partner of the binding complex and (iii) a detectable label bound to or capable of binding to (i) and/or (ii), wherein the first partner(s) and the further partner(s) of the binding complex are capable of forming a reversible bond and wherein an aggregate comprising at least two TCR binding ligands (i), at least one component (ii) and at least one component (iii) is bound via the TCR molecule to said T cell, wherein said T cell is stained, (c) optionally separating said stained T cell from other components of said sample and (d) optionally removing said staining from said T cell by disrupting the reversible bond. The method of the present invention is suitable for reversible staining of cells having a functional receptor molecule, e.g. of T cells, i.e. cells having a functional TCR molecule, wherein said receptor molecule is capable of binding to a receptor ligand, e.g. a peptide/MHC complex. The receptor ligand may be any molecule capable of binding to a receptor molecule present on the cell to be stained, particularly when the receptor ligand and the receptor molecule exhibit a low affinity interaction, e.g. a dissociation constant K d in the range of 10 '2 to 10 "7 M, particularly of 10 "3 to 10 6 M. Thus, a single molecule of the receptor ligand is not capable of forming a stable bond with the receptor on the target cell. In order to provide sufficient avidity for the formation of a stable bond between receptor and receptor ligand at least two or more ligand molecules have to bind cooperatively to the receptor molecules on the target cell. This cooperative binding is accomplished by using ligand molecules conjugated to a first partner of a binding pair which is coupled to the further partner of the binding pair which has two or more, e.g. 3, 4, 5 or more, binding sites for first partner-ligand conjugates. If a single ligand molecule has a very low binding affinity, the number of cooperatively bound ligand molecules has to be increased in order to provide sufficient avidity for the formation of a stable bond. A ligand molecule may be conjugated to one or more first partners of a binding pair. For example, if the ligand molecule is a peptide/MHC complex, a first partner, e.g. a peptide, may be conjugated to an a and/or a β chain of the MHC molecule. More preferably, the ligand molecule comprises at least two first binding partners, for example, a first binding partner may be coupled to the a chain of the MHC molecule and another first binding partner may be coupled to the β chain of the MHC molecule. Alternatively, a sequential arrangement of several first binding partner molecules may be conjugated to a ligand molecule or a subunit of the ligand molecule such as the a and/or β chain of an MHC molecule. Further examples of suitable receptor molecules having low affinity interactions towards their ligands are cell adhesion receptor molecules such as described in (14) and receptors for costimulatory molecules such as described in (15). Still further examples are low affinity antibodies and fragments thereof and artificially engineered binding molecules such as peptides, lipochalins and other aptamers, e.g. from random libraries which bind molecules located on the cell surface and which exhibit low binding affinity but specific binding characteristics. If the receptor ligand comprises an MHC specific peptide, it is preferred that the peptide is a T cell epitope capable of binding to the TCR receptor of the cell to be analysed and to the MHC molecule. The cell is preferably a mammalian cell, e.g. a mammalian T cell, particularly a human cell, e.g. a human T cell, particularly a subpopulation of mammalian T cells having a predetermined antigen- specifity. The peptide comprises a T cell epitope capable of binding to a TCR molecule, preferably to a TCR molecule having a predetermined antigen- specifity. The peptide usually has a length of about 8 to about 25 amino acids and preferably comprises so-called anchor amino acid residues capable of allele-specific binding to a predetermined MHC molecule class, e.g. an MHC class I, an MHC class II or a non-classical MHC class. The MHC molecule is preferably a recombinant soluble MHC molecule, which may be prepared in a bacterial expression system (16) or in an insect cell expression system (17). It is an important feature of the invention that a binding complex, e.g. a binding pair is selected consisting of at least one first partner and at least one further partner wherein the at least further partner has at least two binding sites and more preferably at least four binding sites for the first partner. The bond between the first and the second partner should be reversible, i.e. the bond should be capable of being disrupted under conditions suitable for carrying out the claimed method. Preferably the reversible bond has a K d between 10 "2 and 10 "13 M, more preferably between 10 '3 and 10 '10 M and most preferably between 10 "5 and 10 '8 M as determined under appropriate conditions, e.g. by fluorescence titration (1 1 ). Particularly preferred is a reversible bond having a high K off value, e.g. 5 between 10 '1 sec "1 and 10 '4 sec "1 , especially between 10 '2 sec "1 and 10' 3 sec "1 . It is a further important feature of the present invention that the staining of the target cell, e.g. the T cell, and the subsequent optional steps, namely o the isolation and purification of the stained target cell, e.g. the T cell, and the removal of the staining may be carried out at low temperatures, i.e. at temperatures where substantially no activation and/or signalling events occur, which might result in an alteration of the target cell, e.g. the T cell phenotype. Preferably the staining and the subsequent removal of the 5 staining is carried out at a temperature of < 15°C, more preferably < 10°C and most preferably at about 4°C. The separation of the stained target cell, e.g. the T cell, from other sample components, e.g. unstained T cells may be effected by conventional o methods, e.g. cell sorting, preferably by FACS methods using commercially available systems (e.g. FACSVantage by Becton Dickinson or Moflo by Cytomation), or by magnetic cell separation (e.g. MACS by Miltenyi). The removal of the staining preferably occurs by targeted disruption of the 5 reversible bond between the first and the second partner of the binding complex. This disruption may be achieved by contacting the stained cell with a free first partner of the binding complex or an analog thereof capable of disrupting the bond between the conjugated first partner (i) and the further partner (ii). Preferably, the free first partner is an analog of the o conjugated first partner having a higher affinity to the further partner than the conjugated first partner. More preferably, the free first partner has an affinity which is at least 3 orders of magnitude and particularly at least 5 orders of magnitude higher than the affinity of the conjugated first partner. Particularly the free first partner has a dissociation constant K d which is at least by a factor of 1 x1 0 "3 and especially 1 x10 '5 lower. The label which is used for the detection of stained cells may be any label which is used in diagnostic and analytical methods. Preferably the label does not negatively affect the characteristics of the cells in the method as described. Preferred examples of labels are fluorescent dyes or magnetic labels. The label may be bound to the ligand, e.g. the peptide and/or the MHC molecule, the first partner and/or the further partner of the binding complex. The label may be a direct label, i.e. a label bound to one of the members of the aggregate as specified above. Alternatively, the label may be an indirect label, i.e. a label which is bound to a receptor which in turn is capable of binding to one of the members of the aggregate as specified above. The removal of the staining from the target cell by disruption of the reversible bond between the first and the second partner results in a loss of the cooperative bond between at least two low-affinity ligands and receptor molecules on the target cell. Thus, the aggregate comprising at least two components (i), at least one component (ii) and at least one component (iii) is completeley disrupted. This results in a complete removal of any reagent bound to the target cell, because the bond between the receptor-binding component and the receptor on the target cell is a low- affinity interaction. The principle of the procedure according to the present invention is explained in the following for a preferred embodiment wherein the binding complex is selected from: (a) (i) biotin and (ii) a streptavidin or avidin analog capable of reversible binding of biotin, (b) (i) a biotin analog capable of reversible binding to streptavidin or avidin and (ii) streptavidin or avidin or a streptavidin or avidin analog capable of reversible binding of said biotin analog, and (c) (i) a streptavidin or avidin binding peptide and (ii) streptavidin or avidin or a streptavidin or avidin analog capable of reversible binding of said streptavidin or avidin binding peptide. In an especially preferred embodiment oligomers or polymers of streptavidin or of avidin or of analogs of streptavidin or of avidin may be prepared by the introduction of carboxyl residues into a polysaccharide, e.g. dextran essentially as described in "Noguchi, A., Takahashi, T., Yamaguchi, T., Kitamura, K., Takakura, Y., Hashida, M. & Sezaki, H. (1 992) . Preparation and properties of the immunoconjugate composed of anti-human colon cancer monoclonal antibody and mitomycin C dextran conjugate. Bioconjugate Chemistry 3, 1 32-137" in a first step. Then streptavidin or avidin or analogs thereof are coupled via primary amino groups of internal lysine residues and/or the free N-terminus to the carboxyl groups in the dextran backbone using conventional carbodiimide chemistry in a second step. For the actually especially preferred embodiment the coupling reaction was performed at a molar ratio of about 60 moles streptavidin or Sf/ep-Tactin per mole of dextran. It should be noted, however, that oligomers or polymers of streptavidin or avidin may also be obtained by crosslinking via bifunctional linkers such as glutardialdehyde or by other methods described in the literature. The streptavidin binding peptide is preferably selected from the Strep-tag peptides as described in (9), (10) or in U.S. patent 5,506, 1 21 which are herein incorporated by reference. More preferably, the Strep-tag peptides comprise the amino acid sequence Trp-X-His-Pro-Gln-Phe-Y-Z, wherein X is any desired amino acid and Y and Z either both are Gly, or Y is Glu and Z is Arg or Lys. Especially preferred is the peptide Trp-Ser-His-Pro-Gln-Phe- Glu-Lys (Strep-tagll). In a further preferred embodiment the binding complex may be selected from calmodulin-binding peptides and calmodulin as described in US-A- 5,985,658 or from any other interactions based on the presence of a divalent cation, particularly from peptides binding to a receptor, e.g. the FLAG peptide, which binds to the monoclonal antibody 4E1 1 as described in US-A-4,851 ,341 . The disruption of these binding complexes may be accomplished by metal ion chelation, e.g. calcium chelation, e.g. by addition of EDTA. Principle of procedure: MHC multimers increase the "relative binding avidity" but not the affinity of monovalent TCR-MHC/peptide interactions (Fig. 1 A). Since monomeric MHC/peptide complexes do not stably bind to TCRs, a targeted disruption of MHC multimers into MHC monomers results in rapid dissociation of surface bound TCR ligands. After complete dissociation of MHC/peptide complexes (at temperatures where no activation/signaling events occur, preferable at 4°C), the functional status of the T cell is not changed upon subsequent transfer to cell culture or adoptive in vivo transfer. The generation of prior art MHC multimer reagents is based on specific biotinylation of soluble MHC complexes. Since streptavidin (SA) has four biotin-binding sites, incubation of biotinylated MHC molecules with (fluorochrome-conjugated) streptavidin at a 4: 1 ratio results in the formation of tetrameric MHC reagents (Fig. l B). The stability of the streptavidi biotin bond is high (K d < 1 x10 "13 M, published to be 4x10 "14 M in "Weber, P.C., Wendoloski, J.J., Pantoliano, M.W. & Salemme, F.R. (1 992). Crystallographic and thermodynamic comparison of natural and synthetic ligands bound to streptavidin. J. Am. Chem. Soc. 1 14, 31 97- 3200"), so the reagents are very stable. Addition of biotin (tested up to a concentration of 50 mM) to MHC-streptavidin multimer stained T cells does not significantly affect the stability of the reagents (within a reasonable time window ( ≤ 1 day)). In contrast thereto the method of the present invention is a technique allowing fast and targeted disruption of MHC multimer reagents under physiological conditions for functional isolation of antigen-specific T cells; particularly a substantially complete and fast disruption at low temperatures [preferably at 4°C]. Further, the procedure is substantially non-toxic to T cells, and the substances used are harmless for (clinical) in vivo applications. Peptide sequences (Strep-tags) such as disclosed in U.S. patent 5,506, 1 21 demonstrate binding affinity for the biotin binding site of streptavidin, e.g. with a K d of approx. between 10 "4 and 10 "5 M (9, 10). The binding affinity may be further improved by making a mutations within the streptavidin molecule. Examples of optimized streptavidin muteins (S/rep-Tactins) are described in U.S. patent 6, 103,493 or ( 1 1 ), which are herein incorporated by reference. Preferably, the streptavidin muteins are characterized in that at position 44 of wild-type streptavidin Glu is replaced by a hydrophobic aliphatic amino acid e.g. Val, Ala, lie or Leu, at position 45 an arbitrary amino acid is present, at position 46 an aliphatic amino acid and preferably a hydrophobic aliphatic amino acid is present and/or at position 47 Val is replaced by a basic amino acid e.g. Arg or Lys and in particular Arg. More preferably, Ala is at position 46 and/or Arg is at position 47 and/or Val or lie is at position 44. Most preferably the mutein has a sequence selected from lle 44 -Gly 45 -Ala 6 -Arg 47 or Val 44 -Thr 45 -Ala 46 -Arg 47 ("Sfrep-Tactin" as described in (1 1 )) . The interaction of the Strep-tagll peptide with Strep- Tactin is characterized by a binding affinity with a K d of approx. 10 '6 M (1 1 ) compared to approx. 10 "13 M for the biotin-streptavidin interaction. Biotin, which still binds with high affinity to Sfrep-Tactin (with a K d estimated to be between 10 '10 M and 10 '13 M), competes with Sfrep-tagll for the binding site. According to the present invention, stable MHC multimers for antigen- specific T cell staining are generated based on the interaction of Strep- tagll/SfrepTactin. Thus, it is possible to competitively disrupt multimers in the presence of relatively low concentrations of biotin in a rapid manner, preferably less than 2 h, more preferably less than 1 h (Fig. 2). This system fulfills the required criteria for disruption of MHC multimers (see also above): because of the extreme affinity differences between Sfrep-tagll/SfrepTactin and biotin/SfrepTactin, complete and very fast competitive binding of biotin (even at low temperatures) takes place; biotin (tested up to a concentration of 50 mM) is non-toxic for T cells and does not alter T cell function and small amounts of biotin (vitamin H) are harmless for in vivo applications. We have tested the S.Vep-tagll/SfrepTactin system for both, the generation of MHC multimers and the targeted disruption of multimers bound to the T cell surface by addition of biotin. Alternatively to this approach reversible T cell staining can be achieved by substituting Strep-tags by biotin analogs such as aminobiotin, iminobiotin or desthiobiotin having a lower affinity for streptavidin or avidin compared to biotin. A still further preferred embodiment relates to the competition of free biotin for the binding of biotinylated MHC molecules to Sf ep-Tactin or other streptavidin analogs having lower affinity for biotin. Alternatively, other streptavidin-binding peptides known in the art may be used, e.g. as described by Wilson et al. (Proc.Natl.Acad.Sci.USA 98 (2001 ), 3750-3755). As one key element of the present invention is the fact that the reversible bond can be disrupted in a targeted manner with the addition of small amounts of the physiological compound biotin (vitamin H) which seems neither to be detrimental to living cells nor to alter their functional state, we claim every binding complex which can be selectively disrupted with that compound or analogs thereof. Thus, apart from the binding complexes between streptavidin or avidin or analogs thereof and biotin or biotin analogs or streptavidin- or avidin-binding peptides also other binding complexes are suitable for the purpose of the invention, provided they allow reversible staining of a cell which can be disrupted by the addition of compounds such as biotin or biotin analogs which do not show any detrimental effect to the cell to be stained. For example, an antigen/antibody-binding pair can be applied, particularly if a polymerized or oligomerized antibody is used. Further examples of such binding complexes besides streptavidin/avidin and their analogs and/or antibodies may be so called engineered protein scaffolds for molecular recognition as published in "Skerra A. (2000). Engineered protein scaffolds for molecular recognition. J. Mol. Recognit. 13, 167-187" after being engineered to recognize compounds which may be disrupted in a targeted manner by disruption using biotin or biotin analogs. Further, the present invention relates to a substantially pure target cell population which is defined by the presence of a specific common receptor and which has been purified from a heterogeneous cell population (i.e. heterogeneous relating to the presence or absence of the specific common receptor) using an affinity reagent binding to said common receptor. The target cell population is characterized in that the affinity reagent has been substantially completely removed (preferably below the detection limit) from said receptor. The complete removal of the affinity reagent is accomplished by using the reversibly multimerized low affinity ligand as described above. Thus, a target cell population is provided having a functional status (defined by the common specific receptor) which has not been altered by the purification method. The common specific receptor which defines the target cell population may be any receptor against which a low affinity ligand as described above may be directed. For example, the receptor may be an antigen defining a cell population or subpopulation, e.g. a population or subpopulation of blood cells, e.g. lymphocytes, monocytes or natural killer cells, bone marrow cells or stem cells, e.g. CD34-positive peripheral stem cells. On the other hand, the receptor may also be a marker for tumor cells. 5 Particularly, the present invention relates to a substantially pure antigen- specific T cell population having a functional status which is substantially unaltered by a purification procedure comprising staining the desired T cell population, isolating the stained T cell population from a sample comprising o non-stained T cell population and removing said stain, i.e. the functional status of the T cell population before purification is substantially the same as after the purification. This T cell population may be obtained by the method as described above. More particularly, a T cell population is provided which is substantially free from any binding reagents used for the 5 isolation of the population, e.g. antibodies or TCR binding ligands such as multimeric TCR binding ligands. Finally, the present invention relates to a fusion polypeptide comprising (a) at least one peptide domain which is a first partner of a binding complex o which is capable of forming a reversible bond with at least one further partner of the binding complex and (b) a ligand domain which binds specifically but with low affinity to a receptor on a cell surface. The fusion polypeptide is preferably a genetic fusion, wherein one or several sequentially clustered peptide domains are located on the N- and/or C- 5 terminus of the ligand domain. The fusion polypeptide may also consist of several subunits, wherein each subunit forms a ligand subdomain which bind together to the receptor. In this case each subunit preferably contains at least one peptide domain. In a preferred embodiment the fusion polypeptide comprises (a) a streptavidin-binding peptide and (b) an MHC o molecule, preferably a soluble MHC molecule. For example, the streptavin- binding peptide may be fused to the N-terminus and/or the C-terminus of the σ-chain and/or the β-microglobulin chain of an MHC molecule. For example, fusion polypeptides comprising at least two streptavidin-binding peptides, e.g. fusion polypeptides having a streptavidin-binding peptide bound both to the C-terminus of the σ-chain and the β-chain have been shown to be suitable for the purpose of the present invention. Preferred streptavin-binding peptides are Strep-tag peptides as described above, particularly Sfrep-tagll. Further, the invention relates to nucleic acids encoding a fusion polypeptide as described above, wherein the nucleic acids are preferably located on a recombinant vector, particularly an expression vector allowing expression of the claimed fusion polypeptide in a suitable host cell, e.g. a eukaryotic or prokaryotic host cell. The method of the present invention allows a functional isolation of target cell populations, e.g. of antigen specific T cell populations based on a reversible staining procedure. The original functional status of target cells, e.g. T cells, can be substantially maintained after the identification and purification. Thus, the method of the invention is of broad benefit for basic research and clinical applications. Examples of preferred applications are as follows: Basic research: Direct ex vivo investigation of the functional status of antigen- specific T cell populations. The functional status of T cell populations in vivo is suggested to be highly diverse and dependent on specific in vivo conditions, but because of the lack of appropriate investigative tools, these aspects are only marginally understood. With MHC multimer techniques, epitope-specific T cell populations can be identified and purified independent of their functional status. However, the binding of multimer reagents interferes with subsequent functional assays. Reversible T cell staining e.g. using MHC-Sfrep-tagll/Strep-Tactin reagents is the first technology allowing the direct functional ex vivo investigation of unaltered diverse T cell populations. Purification of antigen-specific T cell populations for highly efficient in vitro expansion. The characterization of T cell populations obtained ex vivo often requires further in vitro expansion to T cell lines or T cell clones. With MHC multimer techniques, single cells or distinct phenotypic subpopulations within a diverse T cell population can be isolated, but the binding of the reagents to the TCR interferes with the efficiency of in vitro expansion. This experimental problem is solved by reversible T cell staining using the reagents of the invention, e.g. MHC-Sfrep-tagll/Strep-Tactin reagents. Purification of antigen-specific T cell populations for adoptive transfer experiments. Many in vivo experiments in immunological research require the adoptive transfer of purified T cells into recipient animals. Both, the purity of the transferred T cell populations and possible changes in the cells that occur during the isolation procedure are concerns in these experimental systems. The highest purity of cell populations is achieved by positive selection methods, but the markers (usually antibodies) used for identification are difficult to remove from the surface of isolated cells and can interfere with the outcome of subsequent in vivo experiments. Reversible T cell staining using the reagents of the invention, e.g. MHC- Sf ep-tagll/Sfrep-Tactin reagents allows the combination of positive selection methods with later removal of the selection marker and might greatly improve adoptive transfer experiments. - TCR-MHC affinity measurements. Diversity of epitope-specific T cell populations is reflected on the level of TCR-MHC/peptide binding affinities. While the measurement of TCR-MHC/peptide binding affinities of T cells is still very difficult, several recent studies indicate that MHC/peptide dissociation rates correlate with relative binding affinities ( 1 2, 1 3) . With the reagents of the invention, e.g. MHC-Sfrep-tagll/Sfrep-Tactin reagents, MHC/peptide complexes can be accumulated on the cell surface of epitope- specific T cells and MHC dissociation rates can be determined after fast monomerization by addition of biotin. This is possible because removal of the Sfrep-Tactin backbone is significantly faster than MHC dissociation rates. Clinical applications: Purification of antigen-specific T cell populations for highly efficient in vitro expansion. Generation of human T cell lines or clones (e.g. pathogen/tumor-specific or autoreactive T cells) is necessary in many areas of clinical research, diagnostics, and immunotherapy. In vitro culture is often limited by difficulties in standardizing conditions for antigen-specific stimulation. Improved strategies for the purification of antigen-specific T cell populations could greatly enhance the efficiency of in vitro expansion, allowing the use of antigen-independent stimulation such as mitogens and anti-CD3. With the invention, e.g. with MHC-Sfrep-tagll/Sfrep-Tactin reagents, antigen-specific T cell populations can be isolated directly ex vivo and expanded in vitro after dissociation of the reagents. This approach is expected to be much more efficient than purification using conventional MHC multimer reagents, as the binding of the reagents negatively interferes with the efficiency of in vitro T cell expansion. Purification of antigen-specific T cell populations from in vitro expanded cell lines or clones for further functional analyses or therapy. In vitro expansion of T cells requires the addition of antigen-presenting cells or feeder cells to the culture. For further functional analysis, and especially for therapeutic applications (e.g. adoptive transfer), it would be helpful to remove these contaminating cells. For positive selection procedures (which usually result in the highest degrees of purity), the selection marker should be removable from the T cell surface, as it might interfere with functional assays or adoptive transfer. If T cells are used for in vivo applications the selection marker must be further removed if it contains substances that could cause clinical complications such as allergic reactions. Reversible T cell staining e.g. using MHC-Sfrep-tagll/Sfrep-Tactin reagents fulfills all these criteria. Ex vivo purification of antigen-specific T cell populations for "direct adoptive immunotherapy". The isolation of antigen-specific T cell populations directly ex vivo followed by immediate transfer of the cells into recipients (without any further in vitro propagation) is of special clinical interest. It is expected that directly isolated cell populations are much more efficient than cultured cells for in vivo applications. Extremely high numbers of in vitro expanded T cells are required for effective adoptive transfers, a phenomenon most likely due to the adaptation of T cells to in vitro culture conditions. An example for an important clinical application for this procedure is the parallel purification and adoptive transfer of EBV- and/or CMV-specific T cell populations during [otherwise] T cell-depleted stem cell transplantations, a protocol which is likely to dramatically reduce the incidence of EBV and CMV-related malignancies in transplant patients. Reversible T cell staining and isolation e.g. using MHC-Sfvep-tagll/Sfrep- Tactin reagents could be an ideal method for these clinical applications. - Functional T cell diagnostics. MHC multimer techniques allow quantification and phenotypic characterization of antigen-specific T cells directly ex vivo. However, binding of multimer reagents to the TCR complicates the use of purified cells in functional assays (e.g. chronic virus infections [HIV, CMV, EBV, HBV, HCV], tumor-specific T cell populations). Reversible T cell staining and isolation e.g. using MHC-Sfrep-tagll/Sfrep- Tactin reagents opens the door for powerful evaluation of antigen-specific T cell status in many clinical situations. Further, the present invention is explained in more detail by the following figures and examples. Fig. 1 : The binding of monomeric and multimeric peptide/MHC conjugates to the TCR of T cells. Due to the binding of the peptide/MHC complex to the TCR, the functional status of the T cells is dramatically altered under physiological conditions (prior art). Fig. 2: By targeted disruption of peptide/MHC multimer binding to the TCR molecules the staining is removed resulting in a purified T cell population having substantially unaltered characteristics (invention). Fig. 3: Interference of MHC multimer staining with T cell function in vitro. (a) LLO 91 . 99 -specific T cells were stained for 2 hours with binding (LLO 91 . 99 /H2-K d ) or non-binding (p60 217 . 225 /H2-K d ) streptavidin-PE tetramer ("tet") reagents at 4°C or 37°C. MHC tetramer staining (upper), TCR expression (middle, filled black graph hidden behind TCR hιgh populations), and annexin V binding (lower) are shown in histogram format, (b) Immunofluorescence staining of LLO 91 . 99 -specific T cells one hour after incubation with LLO 91 . 99 /H2-K d streptavidin-Alexa 546 tetramers (red) at 4°C or 37°C. (c) LLO 91 . 99 - specific T cells stained for 30 min with binding or non-binding tetramer reagents at 4°C, were then washed to remove unbound reagent and transferred to a chromium release assay (effector to target ratio approx. 5:1 ). LLO 91 . 99 peptide concentration and percent specific lysis are shown on the x- and y-axes, respectively. The bars indicate lysis of MHC mismatched target cells (10 '8 M LLO 91 . 99 peptide and no peptide conditions only) by effector cells treated with non-binding MHC tetramer reagents (filled bars) or binding reagents (open bars), (d) LLO 91 . 99 -specific T cells were treated as in (c), transferred into a proliferation assay in the presence of syngeneic irradiated splenocytes, and stimulated in the presence of peptide or various mitogens (x-axis). Fig. 4: Interference of MHC multimer staining with T cell function in vivo. LLO 91 . 99 -specific T cells were stained for 30 min with binding (LLO 91.99 /H2- K d ) or non binding (p60 217 . 225 /H2-K d ) tetramer ("tet") reagents, extensively washed to remove unbound reagent, and transferred into naive recipient mice. Six hours later, mice were infected with 2 x 10 4 Listeria monocytogenes. Numbered viable bacteria (y-axis) determined in the spleen 48 h post-infection are shown following adoptive transfer of 1 x 10 7 LLO 91. 99 -specific T cells (a) or various numbers of LLO 91 . 99 -specific T cells (b) treated with binding (LLO 91 . 99 /H2-K d , open bar) or non-binding (p60 217. 225 /H2-K d , filled bar) tetramer reagents (2 mice per group, standard deviations indicated). Control mice received NaCI only, (c) LLO 91 . 99 /H2-K d tetramer staining (y-axis) after transfer of 1 x 10 7 differently treated T cells (same as in (a)); dot plots are gated on CD8 + T cells, with staining for CD62L, an activation marker downregulated on activated T cells, shown on the x-axis. Fig. 5: Development of a reversible MHC multimer staining technology, (a) LLO 91.99 -specific T cells were stained for 30 min with monomeric or tetrameric ("tet") LLO 91.99 /H2-K d /β 2 m-HSV-tagorp60 217.225 /H2-K d /β 2 m-HSV- tag reagents. Cells were fixed without washing (upper histogram) or after 2 washing steps (lower histogram) with 1 % paraformaldehyde and subsequently stained for remaining HSV-epitope-tag on the cell surface (x- axis), (b + c) LLO 91 . 99 -specific T cells were stained for 45 min with LLO 91 . 99 /K d streptagll/β 2 -HSV-tag SfrepTactin polymers ("poly"), briefly washed to remove unbound reagents, and incubated in the presence (b) or absence (c) of 1 mM d-biotin; all steps were performed at 4°C. Cell aliquots were harvested after different time intervals (indicated above histograms), fixed with 1 % paraformaldehyde, and stained for remaining HSV-epitope-tag on the cell surface (x-axis). Fig. 6: Improvement of MHCstreptagll/SfrepTactin multimer reagents, (a) Model for the stabilization of MHCstreptagll multimer reagents by fusion of a second streptagll sequence to β 2 -microglobulin. (b) LLO 91 . 99 -specific T cells were stained for 45 min with LLO 91 _ 99 /K d streptagll/β 2 -streptagll-HSV- tag SfrepTactin polymers ("poly"), briefly washed to remove unbound reagents, and incubated in the presence (upper histogram) or absence (lower histogram) of 1 mM biotin. All steps were performed 4°C, cell aliquots were harvested after different time intervals (indicated above histograms), fixed with 1 % paraformaldehyde, and stained for remaining HSV-epitope-tag on the cell surface (x-axis). Fig. 7: Reversible MHC multimer staining and T cell function. LLO 91 . 99 - specific T cells were stained for 30 min with binding LLO 91 . 99 /K d streptagll/β 2 m-streptagll-HSV-tag StrepTactin or non-binding (p60 217 . 225 'H2-K d ) multimer reagents and then extensively washed over 4 h in the presence or absence or d-biotin (1 mM, all done at 4°C) prior to assays or adoptive transfer, (a) Staining for recombinant MHC-molecules (detected by epitope-tag staining, x-axis) on differently treated T cells, (b) Cytotoxicity (% specific lysis, y-axis) of differently treated T cells in response to LLO 91 . 99 peptide-coated target cells (peptide titration, x-axis) with an effector to target cell ratio of approx. 5:1 , compare with Fig. 1 c. (c) Proliferation (y-axis) of differently treated T cells stimulated with epitope or various mitogens (indicated on the x-axis) in the presence of syngeneic irradiated splenocytes. Compare with Fig. 1 d. (d) Protection against Lysteria monocytogenes infection following adoptive transfer of 4.1 x 10 6 LLO 91 . 99 -specific T cells treated with binding (LLO 91 . 99 ) or non- binding (p60 217 . 225 ) MHC multimer reagents (indicated to the left). Two groups of mice were injected with cells previously incubated with binding MHCstreptagll reagents (striped bars) and either washed (upper bar, large stripes) or not washed (lower bar, small stripes) in the presence of d-biotin. Control mice received cells treated with binding (open bar) or non-binding (filled bar) conventional MHC tetramer reagents (plus d-biotin wash) or were injected with NaCI only (grey bar). Numbers of viable bacteria (y-axis) were determined 48 h following Lysteria infection. Data are representative of three independent experiments, each with 2 mice per group. Fig. 8: Reversible MHC multimer staining can be transferred to human T cells. HLA*A0201 streptagll fusion proteins were generated and folded in the presence of recombinant human β 2 microglobulin-HSVtag-streptagll and the epitope MART1 27 . 35 to obtain soluble MHC molecules. These were 5 subsequently multimerized with streptactin-polymers and used for staining of a MART 27 . 35 -specific T cell clone (kindly provided by Burkhard Schmidt, Institute of Hematology, Klinikum rechts der Isar, Munich). MART1 27 . 35 - specific T cells were stained for 30 min at 4°C with HLA*A0201 streptagll- Streptactin multimer reagents; after washing, aliquots of the stained cells o were incubated for different periods of time in the presence of 1 mM d- biotin, all done at 4°C. Secondary staining for HSV-epitope was performed as described before (see also legend to Fig. 3); then cells were fixed in 1 % paraformaldehyde and analyzed by flow cytometry. Histograms show fluorescence intensity for HSV epitope tag staining. 5 Examples 1 . Materials and Methods o Generation of H2-Kd Strep-tagll fusion proteins The pET3a/H2-Kd expression vector (5) was mutated by standard PCR techniques to exchange the C-terminal biotinylation site to the Sfrep-tagll sequence. Protein expression was induced in the expression host 5 BL2KDE3) by addition of IPTG and subsequently, inclusion bodies were purified as described before (5) . In vitro refolding and generation of multimeric MHC Sfrep-tagll reagents 0 H2-Kd Strep-Tagil and mouse β 2 -microglobulin (containing a HSV epitope tag at the C-terminus) inclusion bodies were dissolved in 8 M urea and thereafter refolded by rapid dilution into arginine-rich buffer in the presence of high amounts of synthetic MHC binding peptide (here: 0.5 mg/ml LLO 91 . 99 , GYKDGNEYI or p60 217 . 225 , KYGVSVQDI, respectively) as described previously (5) . Soluble MHC complexes were further purified by gel filtration (Superdex 200HR, Pharmacia) and aliquots were stored in liquid nitrogen. For multimerization Sfrep-Tactin polymers were incubated overnight with soluble H2-Kd Sfrep-tagll/LLO 91 . 99 complexes at a molar ratio of 2: 1 or 1 : 1 (2 MHC molecules or 1 MHC molecule per 1 Sfrep-tag binding site). Generation of conventional multimeric MHC reagents Conventional tetramers were generated using PE- or Alexa 546 -conjugated streptavidin (MolecularProbes) . MHC multimer reagents were stored at 2 mg/ml in PBS pH 8.0 containing 0.02% NaAzide, 0.1 mM EDTA, 1 μg/ml pepstatin, and 1 μg/ml leupeptin (5). Generation and staining of T cell lines, functional assays LLO 91 . 99 specific T cell lines were expanded in vitro as described before (1 8) . MHC-multimer staining was performed by incubation of approximately 2 μg H2-Kd Sfrep-tagll multimer reagent per 1 x 10 6 cells at 4°C for 30 min. For epitope-tag staining cells were washed, fixed briefly in 1 % paraformaldehyde, and subsequently stained for anti-HSV tag using an unconjugated primary mouse or rat mAb (e.g. TB067, Novagen) and a secondary anti-mouse or anti-rat-PE antibody (e.g. A85-1 , PharMingen) . Flow cytometry was performed using a FACSCalibur (Becton Dickinson) and FlowJo software. For dissociation experiments stained cells were washed several times (usually 10 times) in 1 mM biotin buffer (PBS, 5% FCS) usually at 4°C. Conventional 51 Cr-release and 3 H-thymidine incorporation assays were performed as described before (1 8). Adoptive transfer experiments LLO 91.99 specific T cells were stained with H2-Kd Sfrep-tagll multimer reagents at 4°C (30 min) and subsequently washed extensively in biotin- free medium. Dissociation by addition of biotin was done as described above (control cells were treated equally in the absence of biotin). Adoptive cell transfer was performed by injection of 1 x 10 7 cells - per mouse per group - into the tail vein of naive BALB/c mice. 48h after i.v. injection with 5 x LD 50 Listeria monocytogenes strain 43251 (2 x 10 4 cells) the number of bacteria per organ (spleen and liver) was measured by plating out serial dilutions of tissue homogenate. 2. Results Interference of MHC multimer staining with T cell function in vitro Phenotypic changes of cell populations have not been reported so far following epitope-specific T cell staining at low temperatures (4°C). Under these conditions, TCR-MHC mediated signalling events become not activated. MHC multimer staining at higher temperatures (e.g. at room temperature (RT), or at 37°C) has been performed to improve the intensity and/or stabilty of the fluorescent signal through epitope-specific internalization of the multimer. In order to analyze the impact of differential treatment of T cells on their phenotype, we compared LLO 91 . 99 -specific T cell lines following staining with LLO 91 . 99 /H2-K d tetramers at 4°C or 37°C. As a negative control, MHC tetramers containing different Listeria epitope (p60 217 . 227 ) were used; these reagents cannot bind to TCRs of LLO 91 . 99 - specific T cells, thus controlling for potential peptide-independent effects. After 2 h of incubation at 4°C, all LLO 91 . 99 -specific T cells exhibited high intensity staining (Fig. 3a, upper histogram), whereas incubation at 37 °C resulted in a reduced and more diffuse staining pattern. These changes correlated with substantial downregulation of surface TCR (Fig. 3a, middle histogram) and increased binding of annexin V, an early marker of apoptosis (Fig. 3a, lower histogram), after staining at 37°C. Immunofluorescence microscopy (Fig. 3) reveals a typical surface-staining pattern of LLO 91 . 99 /H2-K d tetramers after incubation at 4°C, whereas most multimers are internalized at 37°C. No phenotypical changes were detectable using non-binding MHC multimer reagents. To investigate if MHC multimer staining also interferes with T cell function, we compared the cytolytic capacity of differently treated cytotoxic T cells (CTLs). CTLs were stained with specific MHC tetramers or control reagents, then they were extensively washed to remove unbound MHC molecules, and subsequently incubated at 37 °C at a constant effector to target cell ratio in the presence of various concentrations of LLO 91 . 99 peptide. Labelling with CTLs with MHC tetramers dramatically affects their cytotoxicity profile (Fig. 3c): (1 ) reduction in maximum % specific lysis (the extent of reduction varied between experiments, see also Fig. 7b), (2) decreased peptide sensitivity, and (3) increased lysis in the absence of LLO 91.99 peptide addition. The reduction in maximum lysis and decreased peptide sensitivity seems mostly due to TCR internalization and early apoptotic events (see also Fig. 3a). The increase in spontaneous lysis is - at least in part - mediated by 'bystander effects', since also MHC mismatched cell lines are affected (Fig. 3c, bars); MHC tetramer-induced release of cytolytic substances might affect the viability of neighboring target cells. Even more dramatic is the negative impact of MHC multimer staining on further in vitro expansion of epitope-specific T cells. LLO 91 . 99 /H2-K d tetramer treated cells (Fig. 3d) poorly proliferate in response to antigen or mitogenic stimulation, likely due to loss of T cells through MHC multimer-induced overstimulation causing apoptosis (Fig. 3a). Taken together, these findings demonstrate that MHC multimer staining induces substantial phenotypical and functional changes of epitope-specific T cells at physiological temperatures (37°C). Interference of MHC multimer staining with T cell function in vivo In order to investigate whether MHC multimer staining also affects in vivo function, we adoptively transferred differently treated T cells into naive recipient mice and subsequently monitored protection against primary infection with Listeria monocytogenes. In negative control BALB/c mice, high numbers of viable bacteria are detectable in the spleen 48 hours after infection (Fig. 4a, grey bar). Adoptive transfer of 1 x 10 7 LLO 91 . 99 -specific CTLs treated with non-binding control MHC tetramers confers Listeria- specific protection (black bar). However, pretreatment of transferred cells with binding LLO 91 . 99 /H2-K d tetramer reagents results in a dramatic loss of protective capacity. The degree of protection correlates with the number of transferred cells (Fig. 4b); while 1 x 10 7 CTLs pretreated with binding MHC tetramers still confer some significant Δ/sfer/a-specific protection Listeria infection in mice who received 1 x 10 6 cells is indistinguishable from untreated mice. Adoptive transfer of 1 x 10 s control CTLs still mediates significant protection. After cell transfer, much lower numbers of LLO 91 . g9 - specific T cells could be recovered from recipient mice who received MHC multimer-stained cells as compared to control cells (Fig. 4c); the same results were obtained tracking the transferred T cells with the allotypic surface marker thy1 .1 (data not shown). These data reveal a strong negative impact of MHC multimer staining on in vivo cell transfer, both in terms of survival and function. Development of a reversible MHC multimer staining technology MHC multimers increase the "relative binding avidity" but not the affinity of TCR-MHC/peptide interactions. Although epitope-specific T cells demonstrate some detectable interaction with monomeric MHC/epitope molecules when fixed immediately with paraformaldehyde (Fig. 5a, upper histogram), after brief washing all reagent-derived MHC monomers disappear from the cell surface (Fig. 5a, lower histogram, performed at 4°C). Based on these observations, we postulated that targeted disruption of multimers into MHC monomers results in rapid dissocation of surface bound TCR ligand and - if all procedures performed at low temperatures - T cells might maintain the original functional status and phenotype (see model in Fig. 2). Recently, a short peptide sequence (streptagll) has been identified that demonstrates binding affinity (K D approx. 10 "6 M) for the biotin binding-site of a mutated streptavidin molecule, called "StrepTactin" . The molecule d- biotin, which binds with higher affinity to StepTactin (K D < 10 "13 M), effectively competes with streptagll for the binding site. If MHC multimers based on the interaction of steptagll/StrepTactin could be generated for T cell staining, it should thus be possible to competitively disrupt multimers in the presence of relatively low concentrations of d-biotin (see model in Fig. 2) . We generated soluble H2-K d -streptagll fusion proteins; to be able to specifically detect recombinant MHC molecules, an epitope-tag sequence (HSV/gpD, Novagen) was fused to the C-terminus of β 2 -microglobulin. These changes did not affect the binding specifity of stability of in vitro folded MHC reagents (data not shown). MHC-streptagll molecules incubated with StrepTactin-polymers as 'backbone' for multimerization (kindly provided by IBA, Gόttingen, Germany) demonstrated high intensity T cell staining (Fig. 5b), which rapidly decreased after incubation in the presence of d-biotin (1 mM, all at 4°C). Unfortunately, staining with MHCstreptagll/StrepTactin polymers also decreases over time in the absence of d-biotin, indicating instability of the reagents (Fig. 5c). To improve the stability of MHC-streptagll multimers, an additional streptagll was fused to the C-terminus of β 2 -microglobulin (behind the HSV-tag, Fig. 6a). This strategy substantially improved the reagents; LLO 91 . 99 /K d (2x)streptagll StrepTactin-polymer reagents stained LLO 91 . 99 -specific T cells stably over time (Fig. 6b) . Addition of d-biotin (1 mM, all at 4°C) resulted in immediate and complete dissociation of the StepTactin polymer backbone (Fig. 6b and unpublished results). The amount of surface-bound, reagent-derived MHC molecules subsequently decreased rapidly (Fig. 6b). Reversible MHC multimer staining In order to test the effect of reversible MHC multimer staining on T cell function, in vitro expanded LLO 91 . 99 -specific T cell lines were stained with LLO 91 . 99 /K d (2x)streptagll StrepTactin-polymer reagents, and washed over 4 h in cold (4°C) buffer containing d-biotin (1 mM) or no d-biotin. As controls, cells were pretreated with binding (LLO 91 . 99 /H2-K d ) or non-binding (p60 217.227 /H2-K d ) conventional tetramer reagents. No MHC-streptagll multimer-derived MHC/LLO 91 _ 99 molecules could be detected on the cell surface after incubation and washing in the presence of 1 mM d-biotin (Fig. 7a); under these conditions no phenotypic changes, like TCR downregulation or increased annexin V binding, could be observed; d-biotin was tested up to concentrations of 50 mM, and no evidence for toxicity or changes of T cell function were observed (data not shown). Testing of differently treated T cells in cytotoxicity assays (see also Fig. 3c) demonstrated high bystander lysis of MHC-streptagll multimer-stained cells without dissociation (Fig. 7b, open squares); after dissociation of the reagents (open triangles), the cytotoxic T cell function is basically indistinguishable from controls (filled squares). The positive effect of reversible T cell staining is even more evident in proliferation assays. LLO 91 . 99 -specific T cells responded poorly to antigen-specific or mitogenic stimulation when MHC multimer reagents remain on the cell surface (Fig. 7c); after dissociation of surface-bound MHC/LLO 91 . 99 complexes, proliferative responses of the cells were identical to control cells. Maintenance of in vivo function of MHCstreptagll multimer-stained cells was tested in adoptive transfer experiments. Pretreatment of LLO 91.99 - specific T cells with conventional binding MHC tetramer reagents (Fig. 7d, open bar) resulted in significantly reduced protection towards Listeria infection following adoptive transfer. In contrast, after complete dissociation of reagent-derived MHC complexes, the same number of cells conferred almost an identical degree of protection as compared to positive controls. Interestingly, without dissociation of MHC-streptagll multimers prior to adoptive cell transfer, the cells were reproducibly more effective than cells coated with conventional MHC tetramers. Since low levels of d- biotin are found in serum that can disrupt the backbone from MHC- streptagll multimers (data not shown), it is likely that serum d-biotin may cause also some dissociation after in vivo transfer. 3. Discussion Our data show that MHC multimers significantly change the phenotype and function of stained T cells at physiological temperatures. This is in accordance with recent findings by other groups demonstrating strong signaling events mediated through MHC multimer reagents, leading to overstimulation and cell death. To overcome this problem, we developed a reversible staining procedure that allows specific detection of antigen- specific T cells by surface staining at low temperatures. Addition of a competitor resulted in rapid dissassembly of surface bound multimers and subsequent dissociation of monomeric MHC/peptide molecules from the T cell surface. Using this new technique, we find that the functional status of T cells can be maintained after their identification and purification. Several factors are preferably considered in the developement of a reversible T cell staining technique: (1 ) Rapid and complete monomerization should be accomplished at low temperatures [preferably at 4°C]; (2) the procedure should be non-toxic to T cells; and (3) the substances used should be harmless for (clinical) in vivo applications. The MHC-streptagll multimer system fulfills all these requirements: ( 1 ) Because of the extreme affinity differences between streptagll/StrepTactin and d-biotin/StrepTactin, complete and very fast competitive binding is achieved even at low temperatures; (2) d-biotin (vitamin H) in low concentrations is non-toxic for T cells; we tested d-biotin up to a concentration of 50 mM and could not find evidence for toxicity or changes of T cell function; (3) the amounts of d-biotin that might be transferred together with reversibly stained T cells are far lower than d-biotin concentrations found in conventional vitamin supplementations and are therefore unlikely to be harmful. Our data show that the reversible staining procedure maintains the phenotypical and functional status of T cell populations. T cell function is not significantly affected (see Fig. 7). After dissociation of surface bound MHC/peptide complexes, there is no reagent left on the cells. With prior art antibody-based staining and isolation procedures the marker remains on positively stained populations, potentially influencing T cell function and affecting the success of in vivo applications; adverse (e.g. allergic) reactions might be induced through Fc-receptor-mediated mechanisms or the fluorescence marker itself. Reversible MHC multimer staining avoids these potential complications, which we view as further advantage of this technology for in vivo applications. We were particularly impressed by the deleterious effects of conventional MHC tetramer staining on the outcome of adoptive transfer experiments, a problem overcome by the reversible T cell staining procedure described here. Direct ex vivo isolation (e.g. by fluorescence-activated (FACS) or magnetically-activated cell separation) and adoptive transfer of defined antigen-specific T cell populations could now be realized as a very effective therapeutic strategy. For example adoptive transfer of EBV- and/or CMV- specific T cell populations during (otherwise) T cell-depleted stem cell transplantations is likely to reduce the incidence of EBV and CMV-related malignancies in transplant patients. Purification of antigen-specific T cells also plays an important role in the improvement of T cell cloning strategies, especially when purified antigen-specific cells need to be further expanded by effective antigen-independent stimuli (mitogens or anti-CD3). We are in the process of generating human HLA-streptagll fusion proteins and directly fluorochrome- or magnetic-bead-conjugated StepTactin to test potential clinical applications. Furthermore, the basis principle of the reversible multimer-staining procedure might be applicable to other low avidity receptor-ligand interactions. References 1 . Miyahira, Y., K. Murata, D. Rodriguez, J.R. Rodriguez, M. Esteban, M.M. Rodriguez, and F. Zavala. 1995. Quantification of antigen specific CD8 + T cells using an ELISPOT assay. J. Immunol. Methods 181 :45-54. 2. Murali-Krishna, K., J.D. Altman, M. Suresh, D.J.D. Sourdive, A.J. Zajac, J.D. Miller, J. Slansky, and R. Ahmet. 1998. Counting antigen- specific CD8 T cells: a reevaluation of bystander activation during viral infection. Immunity 8:177-187. 3. Manz, R., M. Assenmacher, E. Pfluger, S. Miltenyi, and A. Radbruch. 1995. Analysis and sorting of live cells to secreted molecules, relocated to a cell-surface affinity matrix. PNAS 92:1921 -1925. 4. Altman, J.D., P.A.H. Moss, P.J.R. Goulder, D.H. Barouch, M.G. McHeyzer-Williams, J.I. Bell, A.J. McMichael, and M.M. Davis. 1996. Phenotypic analysis of antigen specific T lymphocytes. Science 274:94-96. 5. Busch, D.H., I.M. Pilip, S. Vijh, and E.G. Pamer. 1998. Coordinate regulation of complex T cell populations responding to bacterial infection. Immunity 8:353-362. 6. Pamer, E., and P. Cresswell. 1998. Mechanisms of MHC class I- restricted antigen processing. Annu. Rev. Immunol. 16:323-358. 7. McMichael, A., and C. O'Callaghan. 1998. A new look at T cells. J Exp Med 187:1367-1371 . 8. Doherty, P. 1998. The numbers game for virus-specific CD8 + T cells. Science 280:227. 9. Schmidt, T., and A. Skerra. 1993. The random peptide library- assisted engeneering of a C-terminal affinity peptide, useful for the detection and purification of a functional Ig Fv fragment. Protein Eng 6:109-122. 5 10. Schmidt, T., J. Koepke, R. Frank, and A. Skerra. 1996. Molecular interaction between the strep-tag affinity peptide and its cognate target, streptavidin. J Mol Biol 9:753-766. o 1 1 . Voss, S., and A. Skerra. 1997. Mutagenesis of a flexible loop in streptavidin leads to higher affinity for the Sfrep-tag II peptide and improved performance in recombinant protein purification. Protein Eng 10:975-982. s 12. Savage, P. A., J.J. Boniface, and M.M. Davis. 1999. A kinetic basis for the T eel receptor repertoire selection during an immune response. Immunity 10:485-492. 13. Busch, D.H., and E.G. Pamer. 1998. T cell affinity maturation by o selective expansion during infection. J Exp Med 189:701-709. 14. Van der Merwe, P. A., M.H. Brown, S.J. Davis, and A.N. Barclay. 1993. Affinity and kinetic analysis of the interaction of the cell adhesion molecules rat CD2 and CD48. EMBO J 12:4945-54. 5 15. Al-Shamkhani A., S. Mallett, M.H. Brown, W. James, and A.N. Barclay. 1997. Affinity and kinetics of the interaction between soluble trimeric 0X40 ligand, a member of the tumor necrosis family, and its receptor OX40 on activated T cells. J Biol Chem 272:5275-82. 0 16. Garboczi D.N., D.T. Hung and D.C. Wiley. 1992. HLA-A2-peptide complexes: refolding and crystallization of molecules expressed in Escherichia coli and complexed with single antigenic peptides. Proc Natl Acad Sci USA 89:3429-3433. 17. Scott C.A., K.C. Garcia, F.R. Carbone, I. A. Wilson, L. Teyton. 1996. Role of chair pairing for the production of functional soluble IA major histocompatibility complex class II molecules. J Exp Med 183:2087-2095. 18. Busch D.H., and E.G. Pamer. 1998. MHC class l/peptide stability: implications for immunodominance, in-vitro proliferation, and diversity of responding CTL. J Immunol 160: 4441 -4448.

Claims: Claims 1 . A method for reversible staining of cells comprising the steps: (a) providing a sample comprising a cell having a receptor molecule, (b) contacting said cell with (i) at least one component specifically binding to said receptor wherein said at least one component is conjugated to at least one first partner of a binding complex, wherein the component and the receptor exhibit a low-affinity interaction; (ii) at least one further partner of said binding complex having at least two binding sites for said first partner and (iii) a detectable label bound to or capable of binding to (i) and/or (ii), wherein the first partner(s) and the further partner(s) of the binding complex are capable of forming a reversible bond and wherein an aggregate comprising at least two components (i), at least one component (ii) and at least one component (iii) is bound via the receptor molecule to said cell, wherein said cell is stained, (c) optionally separating said stained cell from other components of said sample and (d) optionally removing said staining from said cell by disrupting the reversible bond. 2. A method for reversible staining of T cells comprising the steps: (a) providing a sample comprising a cell having a T cell receptor (TCR) molecule, (b) contacting said T cell with (i) at least one TCR binding ligand comprising a TCR binding peptide and an MHC molecule conjugated to at least one first partner of a binding complex, and (ii) at least one further partner of a binding complex having at least two binding sites for the first partner of the binding complex and (iii) a detectable label bound to or capable of binding to (i) and/or (ii), wherein the first partner(s) and the further partner(s) of the binding complex are capable of forming a reversible bond and wherein an aggregate comprising at least two TCR binding ligands (i), at least one component (ii) and at least one component (iii) is bound via the TCR molecule to said T cell, wherein said T cell is stained, (c) optionally separating said stained T cell from other components of said sample, and (d) optionally removing said staining from the T cell by disrupting the reversible bond. 3. The method of claim 1 or 2 wherein step (b), step (c) and step (d) are carried out at a temperature of ≤ 1 5 °C. 4. The method of claim 3 wherein the temperature is about 4°C. 5. The method of any one of claims 1 -4 wherein said cell is a mammalian cell. 6. The method of any one of claims 2-5 wherein said MHC molecule is a soluble mammalian MHC molecule. 7. The method of any one of claims 1 -6 wherein said staining is removed by contacting said stained cell with a free first partner of said binding complex or an analog of said first partner capable of disrupting the bond between the conjugated first partner (i) and the further partner (ii). 8. The method of any one of claims 1 -7 wherein the first free partner is a compound which is not detrimental to the cell to be stained. 9. The method of any one of claims 1 -8 wherein said binding complex is selected from (a) (i) biotin and (ii) a streptavidin or avidin analog capable of reversible binding of biotin, (b) (i) a biotin analog capable of reversible binding to streptavidin or avidin and (ii) streptavidin or avidin or a streptavidin or avidin analog capable of reversible binding of said biotin analog, and (c) (i) a streptavidin or avidin binding peptide and (ii) streptavidin or avidin or a streptavidin or avidin analog capable of reversible binding of said streptavidin or avidin binding peptide. 10. The method of claim 9 wherein said binding complex is selected from (i) the streptavidin-binding peptide Trp-Ser-His-Pro-Gln-Phe-Glu- Lys and (ii) the streptavidin-analogs streptavidin Val 44 -Thr 45 -Ala 46 - Arg 47 ) and/or streptavidin lle 4 -Gly 5 -Ala 6 -Arg 47 . 1 1 . The method of any one of claims 1 -8, wherein the binding complex is selected from partners binding in the presence of a divalent cation. 1 2. The method of any one of claims 1 -1 1 , wherein removal of the staining results in a dissociation of the receptor-binding component from the stained cell. 1 3. Use of the method of any one of claims 1 -1 2 for the isolation of a specific cell population without substantially altering the original functional status thereof. 14. The use of claim 1 3 wherein said cell population is an antigen- specific T cell population. 1 5. The use of claim 14 for the determination of the functional status of an antigen-specific T cell population. 1 6. The use of claim 14 for the purification of an antigen-specific T cell population for in vitro expansion. 1 7. The use of claim 14 for the purification of an antigen-specific T cell population for adoptive transfer or immunotherapy. 1 8. The use of claim 1 4 for T cell diagnostics. 1 9. Reagent kit for reversible staining of cells comprising (i) at least one component specifically binding to a receptor molecule on a cell wherein said at least one component is conjugated to at least one first partner of a binding complex, wherein the component and the receptor exhibit a low-affinity interaction, (ii) at least one further partner of said complex having at least two binding sites for said first partner, and (iii) a detectable lable bound or capable of binding to (i) and/or (ii), wherein the first partner(s) and the second partner(s) of the binding complex are capable of forming a reversible bond. 20. Reagent kit for reversible staining of T cells comprising (i) at least one TCR binding ligand comprising at least an MHC molecule conjugated to at least one first partner of a binding complex, (ii) at least one further partner of a binding complex having at least two binding sites for the first partner of the binding complex and (iii) a detectable label bound or capable of binding to (i) and/or (ii), wherein the first partner(s) and the further partner(s) of the binding complex are capable of forming a high affinity reversible bond. 21 . The kit of claim 20 wherein the TCR binding ligand further comprises a TCR binding peptide. 22. The kit of claim 20 wherein a TCR binding peptide having a desired specificity is provided as a separate part. 23. The kit of any of claims 1 9-22 further comprising a free first partner of said binding complex or an analog of said first partner capable of disrupting the bond between the conjugated first partner (i) and the further partner (ii). 24. Substantially pure target cell population, wherein said cell population is defined by the presence of a specific common receptor and which has been purified from a heterogeneous cell population using an affinity reagent binding to said common receptor, wherein said affinity reagent has been substantially completely removed from said receptor. 25. Substantially pure antigen-specific T cell population having a functional status which is substantially unaltered by the purification. 26. The cell population of claim 24 or 25 obtainable by the method of any one of claims 1 -12. 27. The cell population of claim 24, 25 or 26 which is free from substantial amounts of TCR binding ligands. 28. A fusion polypeptide comprising (a) at least one peptide domain which is a first partner of a binding complex, which is capable of forming a reversible bond with at least one further partner of the binding complex and (b) a ligand domain which binds specifically but with low affinity to a receptor on a cell surface. 29. A fusion polypeptide comprising (a) at least one streptavidin-binding peptide and (b) an MHC molecule.

Inventors: BUSCH DIRK H, WAGNER HERMANN, BUSCH, DIRK, H., WAGNER, HERMANN
Applicants: BUSCH DIRK H, IBA GMBH, WAGNER HERMANN, BUSCH, DIRK, H., WAGNER, HERMANN
Application year: 2001
Main classification: G01N33
IPC codes: A61K35, C07K7, C07K14, C07K19, C12N5, G01N33