IMPROVED MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) MOLECULES

Abstract: Processes for increasing antigen binding capacity by removing an antigen bound to the antigen-binding fragment of a major histocompatibility complex (MHC) molecule or a chain thereof are described. Such processes comprise treating an antigen-bound antigen-binding fragment of the MHC molecule with an unfolding agent to release undesired antigen from the antigen-binding fragment of the MHC molecule and then treating the antigen-free antigen-binding fragment of the MHC molecule or chain with an effective amount of a refolding agent to produce a functional antigen-binding fragment of the MHC molecule. Additionally, compositions of MHC molecules that are substantially free of undesired antigens or that have been bound with desired specific antigens, alone or in combination with pharmaceutically acceptable carriers and/or cytotoxic agents, are described. The present invention has utility for therapeutic administrations, drug screening, and diagnostics.

Description: IMPROVED MAJOR HISTOCOMPATIBILITY COMPLEX fMHC) MOLECULES This invention was made in part with United States government funding of a grant from the National Institutes of Health (NIH Grant Numbers 5R01 Al 13587-13 and 5R37 Al 13587- 15) ; therefore, the government may have certain rights in this invention. BACKGROUND OF THE INVENTION The present invention relates generally to the field of immunology and, more particularly, to processes for removing antigens bound to an antigen-binding fragment of class I and class II MHC molecules and to compositions of MHC molecules that are substantially free of such antigens. Major histocompatibility complex (MHC) molecules are glycoproteins expressed on the surface of cells of higher vertebrates and play a role in immune responses. These molecules were discovered as a result of tissue grafting experiments wherein it was shown that graft rejection is an immune response to foreign antigens on the surface of the grafted cells. In humans, MHC molecules are referred to as HLA (human-leukocyte-associated) antigens because they were first identified in leukocytes. In mice, they are designated H-2 antigens. MHC molecules are divided into two groups, class I and class II, which differ structurally and functionally from each other. In general, the major function of MHC molecules is to bind antigenic peptides and to display them on the surface of cells. These peptides result from an antigen presenting cell (APC) processing an antigen into peptide fragments, which can be as short as 10 to 20 amino acids. Class I MHC molecules are expressed on almost all nucleated cells and are recognized by cytotoxic T lymphocytes, which then destroy the antigen-bearing cells. Class II MHC molecules are expressed primarily on cells involved in immune responses, such as T lymphocytes, B lymphocytes, macrophages, etc. Class II MHC molecules are recognized by helper T lymphocytes and induce proliferation of helper T lymphocytes and amplification of the immune response to the particular antigenic peptide that is displayed. It is thought that an antigenic peptide first forms a complex with either a class I or class II MHC molecule, and this complex fits into a single recognition site in either a cytotoxic or helper T lymphocyte receptor, respectively. In addition to binding antigenic peptides, MHC molecules also can bind with autologous, or "self" peptides. If T lymphocytes then respond to cells presenting "self" peptides, a condition of autoimmunity results. For a general discussion of the function of MHC molecules, see Grey, H.M. , et al. , Scientific American November, 1989:56-64 (incorporated herein by reference for all purposes) . MHC molecule-antigenic peptide complexes are extremely stable. It has been found that the rates of peptide binding (on-rate or association) and unbinding (off-rate or disassociation) are extremely slow, see Buus, S., et al., Proc. Natl. Acad. Sci. USA 83:3968-3971 (1986); Sadegh-Nasseri, S., et al.. Nature 338:274-276 (1989) (these publications are incorporated herein by reference for all purposes) , and that only a small fraction of isolated MHC molecules are able to bind to added peptides, even when such peptides are present in high concentration and for a long incubation time, see Watts, T.H., et al., Proc. Natl. Acad. Sci. USA 83:9660-9664 (1986); Buus, S., et al.. Science 235:1353-1358 (1987); Adorini, L. , et al.. Nature 334:623-625 (1988) (these publications are incorporated herein by reference for all purposes) . This may be due to the fact that many of the MHC molecules still have peptides bound to them, even after the molecules have been isolated, see Demotz, S., et al.. Nature 342:682-684 (1989); Wallny, H.J. , et al.. Nature 343:275-278 (1990) (these publications are incorporated herein by reference for all purposes) . Difficulties in removing peptides bound to MHC molecules have slowed research on, for example, possible drug candidates to combat autoimmune conditions in the case where MHC molecules are bound to autologous proteins and also on screening methods for detecting certain antigenic peptides bound to the molecules. Although peptides have been removed from class II MHC molecules through inducing a change in pH, see Demotz, S., et al.. Nature 342:682-684 (1989) (incorporated herein by reference for all purposes) , research has been centered on the peptides themselves and not the MHC molecules. Protein folding and unfolding has been generally discussed (see. Creighton, T "Proteins: Structures and Molecular Properties, 1984 W.H. Freeman and Company, New York, which is incorporated herein by reference for all purposes) . The unfolding and refolding of proteins, other than MHC glycoproteins, has been reported, see Jaenicke, R. , Prog. Biophys. Molec. Biol. 49:117-237 (1987) (incorporated herein by reference for all purposes) for a review of such work, and the thermodynamic and kinetic mechanisms have been studied and folding intermediates have been characterized, see Harrison, S.C., et al., Proc. Natl. Acad. Sci. USA 83:4028-4030 (1985); Creighton, T.E., Proc. Natl. Acad. Sci. USA 83:5082-5086 (1988); Udgaonkar, J.B., et al.. Nature 335:694-699 (1988); Roder, H. , et al.. Nature 335:700-704 (1988); and Matouschek, Nature 340:122-126 (1989) (all of these publications are incorporated herein by reference for all purposes) . Additionally, the role of disulfide bond formation has been investigated, see Maher, P.A. , et al., Proc. Natl. Acad. Sci. USA 83:9001-9005 (1986); Goldenberg. , D.P., et al.. Nature 338:127-132 (1989); Matsumura, M. , et al.. Nature 342:291-293 (1989) (all of these publications are incorporated herein by reference for all purposes) . MHC molecules are the subject of Ju, U.S. 4,861,589; Sanderson, U.S. 4,478,823; Sanderson, U.S. 4,440,376; and Bach, et al., U.S. 4,265,873 (all of these patents are incorporated herein by reference for all purposes) . SUMMARY OF THE INVENTION In an aspect of the present invention, a process increases the antigen binding capacity of a class I or class II major histocompatibility complex (MHC) molecule or chain thereof. An antigen-bound MHC molecule or chain thereof is treated with an effective amount of an unfolding agent, preferably a reducing agent whereby disulfide bonds of the MHC molecule are cleaved, to release the undesired antigen from the MHC molecule or chain thereof; the unfolding agent and released antigen are removed; and the antigen-free MHC molecule or chain thereof is treated with an effective amount of a refolding agent to produce a functional MHC molecule or chain thereof. A preferred unfolding agent is the reducing agent, dithiothreitol (DTT) . A variety of agents can be used to refold the MHC molecule or chain thereof, thereby increasing the capacity of the MHC molecule or chain to bind antigens, including specific peptide fragments. Additionally, the present invention comprises compositions comprised of MHC molecules or chains thereof that are substantially free of undesired antigens and that may contain specific peptide fragments, if desired. Such compositions can be combined with pharmaceutically acceptable carriers and cytotoxic agents for the treatment of, for example, autoimmune diseases or conditions. Compositions of MHC molecules or chains thereof that are substantially free of antigens can be labelled for assays. Processes to such uses are also described herein. Thus, the processes and compositions of the present invention are useful in therapeutic administrations, drug screening, and diagnostics. Another aspect the invention is a method for removing an antigen bound to an antigen-binding fragment of a major histocompatibility complex (MHC) molecule. The method includes unfolding the antigen-binding fragment of the MHC molecule by treating the antigen-binding fragment with an unfolding agent to release the antigen; separating the released antigen from the unfolded antigen-binding fragment of the MHC molecule; and refolding the unfolded antigen-binding fragment of the MHC molecule with a refolding agent to produce a functional antigen-free antigen-binding fragment of the MHC molecule. A preferred embodiment includes one additional step of removing the unfolding agent from the unfolded antigen-binding fragment after the step of separating the released antigen and before the step of refolding the unfolded antigen-binding fragment. In preferred embodiments, the antigen-binding fragment of the MHC molecule consists of one or more of the following: an MHC molecule; an MHC molecule selected from the group consisting of class I MHC molecules and class II MHC molecules; a binding chain of an MHC molecule; or the antigen- binding fragment of the MHC molecule is selected from the group consisting of α-chains from a class I MHC molecule, α-chains from a class II MHC molecule, 3-chains from a class I MHC molecule, and mixtures thereof. In other preferred embodiments, the antigen-binding fragment of the MHC molecule is a class II MHC molecule in a floppy conformation or a compact conformation. The unfolding agent is typically a reducing agent, or a thiol-containing compound, or the reducing agent is selected from the group consisting of dithiothreitol (DTT) , dithioerythritol, and mercaptoethanol. During the step of unfolding, the antigen-binding fragment of the MHC molecule is treated with DTT having a concentration in the rage from about 1 mM to about 10 mM. The step of unfolding consists of incubating the antigen-binding fragment of the MHC molecule and DTT for a time period of approximately one hour at a temperature of about 37 'C. Typical refolding agents are oxidants such as 0„ 2, K3.Fe(CN)-6,, and HgCl2,. In other preferred embodiments, the step of refolding consists of dialyzing the antigen-binding fragment of the MHC molecule in air-saturated buffer, whereby the unfolded fragment treated with o . The step of refolding consists of treating the unfolded antigen-binding fragment with a refolding agent and a second antigen, wherein the functional antigen-binding fragment of the MHC molecule has the second antigen bound thereto. In preferred embodiments, the method includes the additional step of incubating the functional antigen-binding fragment of the MHC molecule with a second antigen to produce a antigen-binding fragment of the MHC molecule having a second antigen bound thereto. Another aspect of the invention is a method for removing an antigen bound to an antigen-binding fragment of a major histocompatibility complex (MHC) molecule consisting of steps of: unfolding the antigen-binding fragment of the MHC molecule by treating the antigen-binding fragment with an unfolding agent to release the antigen; separating the released antigen from the unfolded antigen-binding fragment of the MHC molecule; refolding the unfolded antigen-binding fragment of the MHC molecule into a refolded functional fragment of the MHC molecule by treating the unfolded antigen-binding fragment with a refolding agent; and binding a desired antigen to the functional antigen-binding fragment of the MHC molecule by treating the functional fragment of the MHC molecule with the desired antigen. Methods for separating an antigen from an antigen- binding fragment of a major histocompatibility complex (MHC) molecule in solution are also included, which consist of treating the fragment of the MHC molecule with an unfolding agent while maintaining the solution at a constant, non-acidic, pH. Also embraced are substantially pure compositions including an antigen-binding fragment of a major histocompatibility complex (MHC) molecule that is substantially free of bound antigen, the MHC molecule being capable of binding to an antigen. In preferred embodiments, the substantially pure composition consists of an MHC molecule selected from the group consisting of class I MHC molecules and class II MHC molecules or the antigen-binding fragment of the MHC molecule consists of a binding chain of a MHC molecule selected from the group consisting of α-chains a class I MHC molecule, α-chains a class II MHC molecule, ,9-chains from a class I MHC molecule, and mixtures thereof. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 demonstrates the effect of reduction with 1 mM and 10 mM dithiothreitol (DTT) and reoxidation on urine class II MHC molecules, IA d . Fig. 1A is a silver-stained SDS gel, wherein: Lane 1: IA d was incubated for 1 hr at 37 "C in the presence of 100 MM FOva(323-339) . The α/3 heterodimeric floppy (F) and compact (C) conformations migrated with apparent molecular weights of 64 and 55 kD, and the separate α- and ,9-chains with molecular weights of 33 and 27.5 kD, respectively. Lane 2: IA d was reduced with 1 mM DTT for 1 hr at 37 β C in the presence of 100 μM FOva(323-339) . Lane 3: The sample of Lane 2 was reoxidized by dialysis in an air-saturated buffer and subsequently incubated for 1 h at 37 β C with 100 μM FOva(323-339) . Lane 4: Identical to Lane 2, but 10 mM DTT were used for reduction instead of 1 mM DTT. Lane 5: The sample of Lane 4 was reoxidized by dialysis in an air-saturated buffer and subsequently incubated for 1 h at 37 # C with 100 μM FOva(323-339) . Fig. IB shows scans for fluorescent peptides bound to different conformations of IA d . The samples of Lanes 1 to 5 in Fig. 1A were scanned for fluorescent peptides on a fluorescence microscope prior to fixation of the gels. It is evident that reduction (Lane 2 and 4) leads to almost complete loss of peptides. Reoxidation (Lanes 3 and 5) restores peptide binding. The binding capacity of all conformations of IA d was significantly higher after reduction and reoxidation, compared to the untreated material (Lane 1) . Fig. 2 demonstrates the unfolding of separate α- and - ■ -chains and shows a silver-stained SDS gel for separate 0-chains (Lanes 1-4) and α-chains (Lanes 6-9) with and without reduction with varying concentrations of dithiothreitol (DTT) , wherein: Lane l: The / 3-chain of IA d after electroelution from preparative SDS gels. The apparent molecular weight was - a - ~> = 27.5 kD. Lane 2: The 3-chain was incubated for 1 hr at 37 β C with 1 mM DTT. Three bands were visible with MW app = 27.5, 28.5 and 30 kD. Lane 3: The 0-chain was incubated for 1 hr at 37 *C with 1 mM DTT. Only one band was seen migrating with MW 8 - 01 - 5 = 30 kD. Lane 4: The 9-chain was incubated for 30 min at 95 β C in the presence of 10 mM DTT. The apparent molecular weight was identical to that in Lane 3. However, aggregates migrating with MW app = 60 and 70 kD were detected. Lane 5: Molecular weight standards. The proteins are: bovine serum albumin, hen egg albumin, carbonic anhydrase, and glyceraldehyde-3-phosphate dehydrogenase. Trypsin inhibitor and lysozyme are not shown on the gel. Lane 6: The α-chain of IA d after electroelution from preparative SDS gels. It migrates with MW app = 33 kD. Lane 7: The α-chain was incubated for 1 hr at 37 *C with 1 mM DTT. Two bands are visible with MW" PP = 33 and 34.5 kD. Lane 8: The α-chain was incubated for 1 hr at 37 β C with 1 mM DTT. Only one band is left migrating with MW app = 34.5 kD. Lane 9: The α-chain was incubated for 30 min at 95 β C in the presence of 10 mM DTT. The apparent molecular weight was identical to that in Lane 8. Fig. 3 demonstrates the effect of reoxidation to form a reassembled IA d a/β heterodimer from separate α- and jS-chains. Fig. 3A shows a silver-stained SDS gel, wherein: Lane 1: Molecular weight standards (see Lane 5 of Fig. 2) . Lane 2: Electroeluted α-chain. Lane 3: Electroeluted 3-chain. Lane 4: α- and β-chains were mixed and incubated for 1 hr at 37 "C. Lane 5: α- and β-chains were mixed and incubated for 1 hr at 37 β C in the presence of 100 μM FOva(323-339) . Lane 6: Identical to Lanes 4, but incubation was for 18 hr. Lane 7: Identical to Lanes 5, but incubation was for 18 hr. Lane 8: α- and / 3-chains were mixed and incubated for 1 hr at 37 # C in the presence of 100 μM FOva(323-339) and l mM DTT. Lane 9: α- and ,9-chains were mixed and incubated for 1 hr at 37 "C in the presence of 100 μM FOva(323-339) and 10 mM DTT. Lane 10: α- and 3-chains were mixed, reduced for 1 hr at 37 β C with 1 mM DTT, and reoxidized. Lane 11: identical to Lane 10, but 10 mM DTT were used for reduction instead of 1 mM DTT. Lane 12: Identical to Lane 10, but the samples were incubated for 1 hr at 37 'C in 100 μM FOva(323-339) after reoxidation. Lane 13: Identical to Lane 11, but the samples were incubated for 1 hr at 37 # C in 100 μM FOva(323-339) after reoxidation. Fig. 3B shows scans from the gel shown in Fig. 3A. In order to quantify the relative protein concentrations, the gel shown in Fig. 3A was scanned on a gel scanner. The positions of the separate chains, the /β heterodimeric compact (C) and floppy (F)conformations, and the homodimeric α- and /- • -chains (α and β ) are given. When the separate chains were isolated and incubated together without previous reduction and reoxidation, they did not reassemble to a/β heterodimers (Lane 6) . Reassembly to the floppy (F) conformation was observed after reduction with 1 mM DTT or 10 mM DTT and reoxidation (Lanes 12 and 13, respectively). A shoulder at the position corresponding to the compact (C) conformation is seen. A considerable amount of protein aggregated upon reoxidation to oligomers. The oligomers smear on the gel at apparent molecular weights higher than floppy (F) conformation. As the concentration of free β-chain was lower than the concentration of free α-chain, it is assumed that most of the oligomers consist of 0-chains. Fig. 3C shows scans for fluorescent peptides from the gel shown in Fig. 3A. The samples not subjected to reduction and reoxidation (Lane 6) bound small amounts of FOva(323-339) . After reduction with 1 mM DTT or 10 mM DTT and reoxidation (Lanes 12 and 13, respectively), the peptide binding capacities of the separate α- and >9-chains was increased by factors of 9 and 11, respectively, as compared to Lane 6. The reassembled floppy (F) conformation was associated with high amounts of peptides. Although β bound peptides (Lane 6), the ,9-chain oligomers did not (Lanes 12 and 13) . Fig. 4 demonstrates the reassembly of separate chains to the a/β heterodimeric floppy (F) and compact (C) conformations and shows a silver-stained SDS gel and western blots, wherein: Lane 1: Silver-stained gel of a preparation of murine IA d . Lane 2: Western blot of the identical sample of Lane 1. Lane 3: Western blot of refolded and reassembled separate chains. The monoclonal antibody 28-16-8S, which recognizes an epitope expressed only by a/β heterodimers, was used for blotting. Although present in high concentrations, the separate chains are not recognized by 28-16-8S (compare Lanes 1 and 2) . Lane 3 shows that the reassembly experiments yielded the floppy (F) and compact (C) conformations. Fig. 5 shows silver-stained SDS gels of purified HLA- A2 and corresponding gel scans for fluorsceinated peptides. In Figs. 5A and 5B, HLA-A2 was incubated for 1 hour at 37 β C with lOυM F-FLU-M1 and subject to SDS/PAGE omitting prior reduction and boiling the sample. Prior to fixation the gel was scanned on a fluorescence microscope for fluoresc ' einated peptides associated with HLA-A2. The gel was then stained with silver. The silver stained gel (Fig. A) is shown above the scan fluorescence (Fig. B) . The heavy chain migrates with an apparent molecular mass of 43 kD. The fluorescence peak coincides with the position of the heavy chain. Fig. 5C is identical to Fig. 5B, but a fluoresceinated peptide representing amino acids 88-104 of chicken ovalbumin was used instead of F-FLU-Ml. Fig. 5D is identical to Fig. 5B, but the heavy chain has been isolated by electoelution and incubated with 25 mM Tris-HCl/0.2 M glycine, pH 8.2/0.1% SDS. Figs. 5E and 5F are identical to Figs. 5A and 5B, respectively, but the sample was reduced with 10 mM DTT for 1 hour at 37 • C before F-FLU-Ml was added, and the incubation was extended for 1 hour. The apparent molecular mass is 45 kD. They heavy chain dimer at 88 kD disassembled upon reduction. Two bands with apparent molecular mass of 53 and 62 kD appeared, which do not bind peptide. This material may have originated from aggregates of masses higher than 200 kD in the absence of reducing agents (see Fig. 5B) . Fig. 5G is identical to Fig. 5F, but reduction was for 1 hour at 90 • C. Incubation with F-FLU-Ml was at 37*C as above. Fig. 5H is identical to Fig. 5F, but the sample was made 2% SDS in addition to DM before reduction. Figs. 51 and 5J are identical to Figs. 5A and 5B, respectively, but the sample was reduced with 10 mM DTT for 1 hour at 37"C and subsequently dialyzed in air-saturated buffer. The sample was incubated with 10υM F-FLU-Ml for 1 hour at 37"C. The reoxidized sample bound F-FLU-Ml about as well as the untreated sample, but the protein concentration was only 20%, as determined by densitometry of the silver stained gel. Fig. 5K is identical to Fig. 5J, but the HLA-A2 was incubated with 10 μM F-FLU-Ml and 10 μM F-FLU-Ml and 10 μM FLU- MI. The peptides were mixed prior to incubation. Fig. 5L is identical to Fig. 5J, but HLA-A2 was incubated with 10 μM F-FLU-Ml and 100 μM FLU-MI-(56-86) . DESCRIPTION OF THE PREFERRED EMBODIMENTS Structurally, MHC molecules are found in vivo as membrane-bound, heterodimeric glycoproteins. Class I MHC molecules consist of two polypeptide chains, a large α-chain and a smaller β -miσroglobulin chain. The α-chain is about 345 amino residues and has three extracellular globular regions, α , α , and α , with a hydrophobic amino acid sequence near its carboxyl terminus by which it is anchored in the plasma membrane. There are two intrachain disulfide bonds formed from cysteine residues, one in the α region and one in the α region. The class I MHC molecule α-chain and the β -microglobulin chain are noncovalently associated, and the latter is an extracellular, nonglycosylated protein 96 amino acids in length with one intrachain disulfide bond. It is thought that the antigen binding site is formed from the α and α 2 domains of the class I MHC molecule, which are highly polymorphic. Class II MHC molecules are similar to class I MHC molecules in that they also are heterodimers with four extracellular domains. Class II MHC molecules have an α-chain and a 3-chain of approximately equal length, and each chain has two domains designated α , a , β , and β . Disulfide bonds are present in the α 2 , β , and β 2 domains. In vivo, the α 2 and β 2 regions are anchored in the plasma membrane and appear to be highly conserved immunoglobulin-like domains. The amino-terminal domains of the α- and ,5-chains, α χ and ,9 , respectively, are located farthest from the cell membrane and are highly polymorphic. The interaction of these two domains forms the antigen binding site of a class II MHC molecule. For a general summary of the structure of MHC molecules as well as a discussion of their genetics, see Chapter 23 (The Generation of Immunological Specificity) in J. Watson, et al., Molecular Biology of the Gene. Benjamin/Cummings, Menlo Park (4th ed. 1987) ; Chapter 18 (The Immune System) in B. Alberts, Molecular Biology of the Cell. Garland, New York (2d ed. 1989); and Grey, H.M. , et al.. Scientific American November, 1989:56-64 (these publications are incorporated herein by reference for all purposes) . Two distinct a/β heterodimeric conformations have been identified for class II MHC molecules, see Rothenhausler, B., et al., Proc. Natl. Acad. Sci. USA 87:352-354 (1990) (incorporated herein by reference for all purposes) . The antigenic peptide forms a very stable complex with the MHC molecule, and this complex fits within a single recognition site in a T lymphocyte receptor. For discussion of T lymphocyte receptors, see, e.g.. Rothbard, J.B., et al., EMBO Journal 7:93-100 (1988); Townsend, A., et al., Ann. Rev. Immunol. 7:601-624 (1989); Rothbard, J.B., et al.. Cell 52:515-523 (1988); Lamb, J.R. , et al., EMBO Journal 6:1245-1249 (1987); and Bodmer, H.C., Cell 52:253-258 (1988) (all of these publications are incorporated herein by reference for all purposes) . As discussed above, in class I MHC molecules, the α and α 2 regions may be responsible for binding whereas in class II MHC molecules the α and β domains are involved. In both instances, the binding region appears to be a single site that is located in a groove wherein the sides are formed by α-helices and the platform is formed by 3-strands. For a hypothetical model of the antigen binding site of class II MHC molecules, see Brown, J.H., et al.. Nature 332:845-50 (1988) (incorporated herein by reference for all purposes) . See also Chen, B.P., Nature 337:743-745 (1989) for a discussion of the direct binding of influenza peptides to class I MHC molecules. Peptide binding occurs in both the compact (C) and floppy (F) conformations of the class II MHC a/β heterodimer; separate and isolated α-and 3-chains can specifically bind antigenic peptides as well, see Rothenhausler, B. , et al., Proc. Natl. Acad. Sci. USA 87:352-54 (1990) (incorporated herein by reference for all purposes) . In addition to binding antigenic peptides, class II MHC molecules bind with autologous, or "self" peptides, see Buus, S., et al., Science 242:1045-1047 (1988); Demotz, et al., Nature 342:682-684 (1989) (these publications are incorporated herein by reference for all purposes) . The stability (that is, long lifetime) of the MHC molecule-antigenic peptide complex is reflected in the extremely slow off-rate of the peptide to form a free MHC molecule. This feature promotes functioning of MHC molecules in the cell. It is known that antigenic peptides are bound to MHC molecules in an intracellular compartment, then the MHC molecule-antigenic peptide complex is transported to the cell surface, where it "waits" for a specific T lymphocyte. If the complex were not extremely stable (that is, if the antigenic peptide was not trapped or bound with a slow off-rate) , it would disassociate during export or when it was in the plasma membrane waiting for the specific T lymphocyte. It is hypothesized herein, but should not be limiting to the present invention, that folding of the MHC molecule, assembly and peptide binding may take place in the same cellular compartment. This theory is consistent with the finding discussed above that peptides may bind specifically to separate α- or /3-chains, see Rothenhausler, B., et al., Proc. Natl. Acad. Sci. USA 83:352-354 (1990) (incorporated herein by reference for all purposes) . The biological task of MHC molecules is to bind a variety of antigenic peptides and to retain each of them for extended periods of time. When the mechanism of peptide-MHC assembly is ultimately understood, the term "trapping site" may be more appropriate than "binding site." The present invention includes processes for increasing antigen binding capacity of a class I or class II MHC molecule or chain thereof. It also includes methods for removing bound antigens from an antigen-binding fragment of peptide-MHC molecule complexes. These methods comprise the steps of treating an antigen-bound MHC molecule or chain with an effective amount of an unfolding agent, preferably a reducing agent such as dithiothreitol that cleaves disulfide bonds in the MHC molecule, to release the undesired antigen from the MHC molecule or chain; removing or separating the released antigen from the unfolded MHC molecule; and treating the antigen-free MHC molecule or chain with an effective amount of a refolding agent, preferably an oxidizing agent to reform the disulfide bonds, under suitable conditions whereby a functional MHC molecule or chain is produced. In one embodiment, desired specific antigens are then added to the MHC molecules. Both the a/β heterodimer and the separate α- and /3-chains resulting from the present processes bind significantly higher amounts of antigen after having been unfolded and refolded, as compared to an untreated MHC molecule or chain thereof. As used herein, the antigen-binding fragment of a major histocompatibility complex is the part of the MHC molecule that binds a antigen. The antigen-binding fragment may be the whole of a MHC molecule, or it may be a portion of an MHC molecule. Examples of antigen-binding fragment suitable for use with the invention include the following: class I MHC molecules and class II MHC molecules. Other examples include individual chains such as α-chains from a class I MHC molecule, α-chains from a class II MHC molecule, β-chains from a class I MHC molecule, and mixtures thereof. Further examples include MHC fragments that bind an antigen that are produced by cleavage of α-chains from a class I MHC molecule, α-chains from a class II MHC molecule, ,9-chains from a class I MHC molecule. For example, proteolytic cleavage of a portion or all of a non- binding domain to produce a fragment capable of binding an antigen is suitable for use in the invention. MHC molecules can be either from class I or class II or mixtures thereof. Additionally, separate class I α-chains or class II α- or 9-chains or mixtures thereof can be used alone or in combination with whole MHC molecules. For example, the invention methods may be applied to the α-chain of the class I MHC molecule, HLA-A2. Truncated MHC molecules wherein the membrane-bound carboxyl terminals of the chains have been removed can also be used. The MHC molecules can be either of the compact (C) conformation or of the floppy (F) conformation or a mixture thereof. Given the variety of specificities found in MHC molecules and their chains, the type of MHC molecule(s) or chain(s) thereof that are selected will depend upon the research project or therapeutic or diagnostic regime employed. Sources of MHC molecules are familiar to those skilled in the art and include affinity chromatography using cell lines or recombinant DNA techniques. As an example, but not limiting to the present invention, murine IA d , which is a class II MHC molecule, can be affinity purified from cells using an antibody that recognizes epitopes expressed only by a/β heterodimers. As another example, but not limiting to the present invention, HLA-A2, which is a class I MHC molecule, can be purified as described in Chen et al. Nature 337:743-745, which is incorporated herein by reference for all purposes. For other discussion of MHC purification, see, e.g.. Watts, T.H., et al., Ann. Re τ TWVP"! - 5:461-475 (1987); Watts, T.H., et al., Proc. Natl. Acad. Sci. USA 81:7564-7568 (1984); Watts, T.H., et al., Proc. Natl. Acad. Sci. USA 82:5480-5484 (1985) (all of these publications are incorporated herein by reference for all purposes). See also Saito, et al., U.S. 4,874,845 (incorporated herein by reference for all purposes) , for a discussion of the recombinant techniques and T lymphocyte receptors. Suitable MHC molecules for use with the invention can be expressed as is known in the art. Generally, the nomenclature used hereafter and the laboratory procedures in recombinant DNA technology described below are those well known and commonly employed in the art. Standard techniques are used for DNA and RNA isolation, amplification, and cloning. Generally enzymatic reactions involving DNA ligase, DNA polymerase, restriction endonucleases and the like are performed according to the manufacturer's specifications. These techniques and various other techniques are generally performed according to Sambrook et al. , Molecular Cloning - A Laboratory Manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, 1989. Other general references are provided throughout this document. The procedures therein are believed to be well known in the art and are provided for the convenience of the reader. DNA constructs will typically include an expression control DNA sequence, including naturally-associated or heterologous promoter regions, operably linked to protein coding sequences. Preferably, the expression control sequences will be eukaryotic promoter systems in vectors capable of transforming or transfecting eukaryotic host cells. Once the vector has been incorporated into the appropriate host, the host is maintained under conditions suitable for high level expression of the nucleotide sequences, and the collection and purification of the proteins. Human MHC and immunoglobulin constant region DNA sequences can be isolated in accordance with well known procedures from a variety of human cells. For instance, immortalized B cells are suitable for isolating immunoglobulin sequences. MHC cDNA and genomic clones from a variety of cells have been extensively characterized (see, e.g.. Paul, Chapter 17, references 18-33, all of which are incorporated herein by reference) . Typically, traditional screening of a cDNA library prepared from RNA isolated from appropriate cells is used. PCR amplification of the desired sequences can also be used (See, PCR Protocols. Innis et al., eds. Academic Press, 1990). Suitable source cells for the DNA sequences and host cells for expression and secretion can be obtained from a number of sources, such as the American Type Culture Collection ("Catalogue of Cell Lines and Hybridomas," Sixth edition (1988) Rockville, Maryland, U.S.A., which is incorporated herein by reference for all purposes) . It is well known that native forms of "mature" MHC glycoproteins and immunoglobulins will vary somewhat in terms of length by deletions, substitutions, insertions or additions of one or more amino acids in the sequences. Thus, both components are subject to substantial natural modification, yet still capable of retaining their respective activities. Modified protein chains can also be readily designed and manufactured utilizing various recombinant DNA techniques well known to those skilled in the art. For example, the chains can vary from the naturally-occurring sequence at the primary structure level by several amino acid substitutions, terminal and intermediate additions and deletions, and the like. Alternatively, polypeptide fragments comprising only a portion (usually at least about 60-80%, typically 90-95%) of the primary structure may be produced. In particular, it is noted that like many genes, the immunoglobulin and MHC genes contain separate functional regions, each having one or more distinct biological activities. Thus, for example, the immunoglobulin component may be modified so as to retain certain functions (e.g.. complement fixation activity) , while exhibiting lower immunogenicity. In general, modifications of the genes encoding either the MHC or immunoglobulin component of a protein may be readily accomplished by a variety of well-known techniques, such as site-directed mutagenesis (see. Gillman and Smith, Gene 8:81-97 (1979) and Roberts, S. et al.. Nature 328:731-734 (1987), both of which are incorporated herein by reference for all purposes) . The nucleic acid sequences capable of ultimately expressing the desired protein can be formed from a variety of different polynucleotides (genomic or cDNA, RNA, etc.) by a variety of different techniques. (See. European Patent Application NOS. 85102655.8, 85305604.2, 84302368.0 and 85115311.4, as well as PCT Application Nos. GB85/00392 and US86/02269, all of which are incorporated herein by reference) . As stated previously, the nucleotide sequences encoding the MHC and immunoglobulin components will be expressed in hosts after the sequences have been operably linked to an expression control sequence (i.e., positioned to ensure the translation of the structural gene) along with other sequences (e.g.. enhancers, polyadenylation sites, etc.) necessary for efficient transcription and translation of the desired sequences. This collection of sequences is referred to here as an "expression cassette" which is typically replicable in the host organisms either as episomes or as an integral part of the host chromosomal DNA. Commonly, expression vectors comprising the expression cassette will contain selection markers, e.g.. tetracycline or neomycin, to permit detection of those cells transformed with the desired sequences (see, e.g. f U.S. Patent 4,704,362, which is incorporated herein by reference for all purposes) . In general, prokaryotes are used for cloning the protein nucleotide sequences. E. coli is particularly useful for cloning the nucleotide sequences of the present invention. Other microbial hosts suitable for use include bacilli, such as Bacillus subtilus . and other enterobacteriaceae, such as Salmonella , Serratia. and various Pseudomonas species. In these prokaryotic hosts, one can also make expression vectors, which will typically contain expression control sequences compatible with the host cell (e.g.. an origin of replication) . In addition, any of a variety of well-known promoters will be present, such as the lactose promoter system, a tryptophan (trp) promoter system, a beta-lactamase promoter system, or a promoter system from phage lambda. The promoters will typically control expression, optionally with an operator sequence, and have ribosome binding site sequences and the like, for initiating and completing transcription and translation. Other microbes, such as yeast may also be used for expression. Saccharomyces is a preferred host, with suitable vectors having expression control sequences, an origin of replication, termination sequences and the like as desired. Typical promoters include 3-phosphoglycerate kinase and other glycolytic enzymes. Inducible yeast promoters include, among others, promoters from alcohol dehydrogenase 2, isocytochrome C, and enzymes responsible for maltose and galactose utilization. When constructing vectors for use in yeast, the plasmid YRp7 can be used, Stinchcomb, et al., Nature. 282: 39 (1979), which is incorporated by reference. This plasmid contains the trpl gene which is a selectable marker for a mutant strain which lacks the ability to grow on media containing tryptophan. The presence of the trpl gene allows transformed mutant cells to grow on selective media and to be identified. Mammalian tissue cell culture will typically be used to produce the polypeptides of the present invention (see, Winnacker, "From Genes to Clones," VCH Publishers, N.Y., N.Y. (1987) , which is incorporated herein by reference) . Eukaryotic cells are actually preferred, because a number of suitable host cell lines capable of secreting intact immunoglobulins have been developed in the art, and include the CHO cell lines, various COS cell lines, HeLa cells, myeloma cell lines such as SP2/0 or lymphoma lines such as BW5147, etc. Expression vectors for these cells can include expression control sequences, such as an origin of replication, a promoter, an enhancer (Queen, C. et al., Jpτττ>ιpr>1. Rev. 89:49-68 (1986), which is incorporated herein by reference) , and necessary processing information sites, such as ribosome binding sites, RNA splice sites, polyadenylation sites, and transcriptional terminator sequences. Preferred expression control sequences are promoters derived from immunoglobulin genes, SV40, Adenovirus, Bovine Papilloma Virus, hCMV early enhancer/promoter (Boshart et al.. Cell 41:521-530 (1985)), SRα (Takebe et al., Mol. and Cell Biol.. 8:486-472 (1988)) and the like. Eukaryotic transcription can be increased by inserting an enhancer sequence into the vector. Enhancers are cis-acting sequences of between 10 to 300bp that increase transcription by a promoter. Enhancers can effectively increase transcription when either 5' or 3' to the transcription unit. They are also effective if located within an intron or within the coding sequence itself. Typically, viral enhancers are used, including SV40 enhancers, cytomegalovirus enhancers, polyoma enhancers, and adenovirus enhancers. Enhancer sequences from mammalian systems such as the mouse immunoglobulin heavy chain enhancer are also commonly used. Mammalian expression vector systems will also typically include a selectable marker gene. Examples of suitable markers include, the dihydrofolate reductase gene (DHFR) , the thymidine kinase gene (TK) , or prokaryotic genes conferring drug resistance. The first two marker genes require the use of mutant cell lines that lack the ability to grow without the addition of thymidine to the growth medium. Transformed cells can then be identified by their ability to grow on non-supplemented media. Examples of prokaryotic drug resistance genes useful as markers include genes conferring resistance to neomycin, G418, xpgt and hygromycin. As discussed above, the proteins may also comprise protease recognition sites between the MHC and immunoglobulin components. Thus, nucleotides encoding the appropriate protease recognition site will be included. For Factor Xa, the recognition site is IleGluGlyArg. For collagenase, the recognition site is ProLeuGlyProArg. The vectors containing the nucleotide segments of interest (e.g.. the MHC chain and immunoglobulin encoding sequences and expression control, sequences) can be transferred into the host cell by well-known methods, depending on the type of cellular host. For example, calcium chloride transfection is commonly utilized for prokaryotic cells, whereas calcium phosphate treatment or electroporation may be used for other cellular hosts. Other methods used to transform mammalian cells include the use of Polybrene, protoplast fusion, liposomes and micro injection. See, generally. Sambrook et al., supra. Once expressed, the whole proteins, or individual chains of the present invention can be purified according to standard procedures of the art, including ammonium sulfate precipitation, fraction column chromatography, gel electrophoresis and the like, (See, generally. Scopes, R. , Protein Purification. Springer-Verlag, N.Y. (1982) , which is incorporated herein by reference.) Once purified, partially or to homogeneity as desired, the polypeptides may then be used therapeutiσally or in developing and performing assay procedures, immunofluorescent staining, and the like. (See. generally. Immunological Methods. Vols. I and II, Eds. Lefkovits and Pernis, Academic Press, New York, N.Y. (1979 and 1981), which are incorporated herein by reference.) Proteins of the present invention can find a wide variety of utilities in vitro, in addition to treating autoimmune and other immunological disorders. By way of example, they can be utilized for diagnostic purposes, T cell typing, for isolating or labeling specific cells, and the like. For any of these purposes, the proteins may be labeled or unlabeled. Unlabeled proteins can be used in combination with labeled antibodies that are reactive with the immunoglobulin portion of the molecule. Antibodies specific for human immuno¬ globulin constant regions are well known in the art. See. generally. Harlow and Lane, Antibodies: A Laboratory Manual (1988) . Labeled protein A or protein G may also be used for this purpose. These proteins are normal constituents of the cell walls of streptococcal bacteria. They exhibit a strong non-immunogenic reactivity with immunoglobulin constant regions from a variety of species. See, generally Kronral, et al., J. Immunol.. 111:1401-1406 (1973), and Akerstrom, et al., J. Immunol.. 135:2589-2542 (1985), all of which are incorporated herein by reference. Alternatively, the protein can be directly labeled. A wide variety of labels may be employed, such as radionuclides, fluorescent tags, enzymes, enzyme substrates, enzyme cofactors, enzyme inhibitors, chemiluminescent compounds, bioluminescent compounds, etc. Those of ordinary skill in the art will know of other suitable labels for binding to the protein, or will be able to ascertain such using routine experimentation. The binding of these labels can be done using standard techniques common to those of ordinary skill in the art. Antigen, as used herein, refers to any substance that can fit into and bind to the binding site of a class I or class II MHC molecule or a chain derived thereof. Usually this will be a peptide fragment but, depending upon the antigen encountered, it can include whatever an antigen presenting cell (APC) puts on its surface. For example, a polysaccharide fragment may be presented. Antigens can be either foreign or "self" (autologous) . In the processes of the present invention, an antigen-bound MHC molecule or a chain thereof is treated with an effective amount of an unfolding agent. An effective amount of a unfolding agent is that amount necessary for the MHC molecule or chain to release substantially all of the bound antigen. The effective amount will vary depending upon the selected unfolding agent, the concentration of MHC molecules. the reaction conditions, etc. In general, an amount that releases greater than 70% of bound antigens is preferred, with an amount that releases greater than 90% of bound antigens being more preferred, and an amount that releases greater than 99% being most preferred. The unfolding (chaotropical) agent includes any chemical or condition, or combination thereof, that induces release of an antigen from a class I and/or class II MHC molecule and/or chain derived thereof. Such release can occur through modification of the structure and flexibility of the MHC molecule or chain. Reduction of the intrachain disulfide bonds can release the antigen, in which case the unfolding agent is referred to as a reducing agent. Additionally, a relaxing of the secondary structure of the molecule or chain can release the antigen. In the latter situation, the disulfide bonds may remain intact, but the integrity of the molecule or chain is disturbed so that antigen may dissociate from the molecule or chain. Preferred unfolding agents that reduce disulfide bonds are dithiothreitol (DTT), dithioerythritol, and mercaptoethanol; with DTT most preferred. Other reducing agents NaBH 4 , LiAlH 4 , H 2 (used catalytically) , P (red) , Zn (in dilute acid), hydroguinone, and (Co,H5_)J3P (in water). Many other reducing agents will be apparent to one skilled in the art and can be used singularly or in combination. Reaction conditions will vary widely depending upon the unfolding agent selected and its effective amount. These conditions include temperature; pH; agitation, if necessary; time period; etc. Those skilled in the art will recognize that by adjusting one reaction parameter, other parameters may need to be modified. For DTT, a concentration range from about 1 mM to about 10 mM is preferred with a treatment time period of approximately 1 hour at a temperature of about 37 β C being most preferred. Obviously, lower concentration ranges of DTT, as well as for other unfolding agents, may require longer treatment periods, such as from approximately 6 to 8 hours or even longer, and temperature ranges may also vary. Using the procedures described in the Experimental section below, it can be determined whether the unfolding agent has unfolded the MHC molecule enough to release substantially all of the bound antigen. It is not necessary to completely unfold and refold the MHC molecule in order to release antigen bound to the MHC molecule and increase its MCH molecule's antigen binding capacity. As used herein, "unfolding" of a protein is used to denote a process that weakens interactions within the protein causing flexibility in protein structure. The unfolded state may persists until "refolding" occurs. Further, the conditions under which a MHC molecule unfolds depends of its initial conformation. As shown in the Examples below, the floppy conformation (F) of a murine IA d class II MHC molecule disassembled upon reduction with DTT and was restored from the separate chains upon reoxidation, but the compact conformation (C) remained unchanged under the same conditions. However, the compact (C) conformation released all of its antigenic peptides upon reduction, and bound 2.5-fold the amount of FOva(323-339) , which is a small peptide (17 amino acids) of chicken ovalbumin that binds MHC molecules, after reoxidation (see Table I below) . The disulfide bonds of compact (C) conformation may be reduced by DTT, but yet the protein may not unfold.Complete unfolding, or unfolding past a certain point, may result in irreversible damage to the function of the MHC molecule. It is known that the formation of disulfide bonds contributes considerably to the free energy stabilizing secondary structures, see Matsumura, M. , et al.. Nature 342:291-29 (1989) (incorporated herein by reference for all purposes) , but, on the other hand, much of stabilization is determined by noncovalent interactions. Because one a/β heterodimeric conformation may unfold upon reduction whereas the other conformation may be stable, MHC molecules can serve as a model for studying the stability of proteins. The fact that compact (C) MHC molecules may release antigens without disassembly or complete unfolding can be hypothesized, but not limiting to the present invention, as resulting from an increased flexibility, despite an unaltered mean structure, due to reduced disulfide bonds that may allow the antigen to disassociate with a faster off-rate than an MHC molecule having oxidized disulfide bonds. If the MHC molecule is rigid, i.e., with oxidized disulfide bonds, the antigen disassociates at an extraordinarily slow rate. Unfolding agents of the present invention also include agents that do not reduce disulfide bonds but sufficiently unfold a portion of the MHC molecule or chain thereof and allow release of the bound antigen. Such agents include urea, guanidinium chloride, thiocyanate, detergents (such as SDS, dodecyl-,9-D-maltoside (DM), TritonX 100, Nonidet P40) , low pH (< 3) (see Demotz, S., et al.. Nature 342:682-684 (1989) (incorporated herein by reference for all purposes)), high pH (> 12) , and elevated temperatures. Detergents serve to stabilize membrane-bound proteins such as MHC molecules in solution. Elevated temperatures are particularly effective when combined with the above detergents and agents that induce low pH. Such agents can be used singularly or in combination. Simultaneous use of such unfolding agents that do not reduce disulfide bonds with reducing agents is also an effective means to unfold MHC molecules and chains thereof. An example of such a combination is the use of SDS or urea with DTT. Additionally, physical agents, such as a change in pH can also be combined. Once the antigen-bound MHC molecule or chain thereof has been treated with an unfolding agent, the unfolding agent and the released antigen are removed, or separated from the unfolded MHC molecule. This can be accomplished by a variety of means, including dialysis, gel filtration, chromatography (such as ion exchange or affinity), electrophoresis, ultrafiltration, ultracentrifugation, chemical, or enzymatic reactions (including neutralization) , or combinations thereof. Next, the unbound MHC molecule or chain thereof is treated with an effective amount of a refolding agent whereby a functional antigen-binding fragment of the MHC molecule is produced. The term "functional antigen-binding MHC molecule" or "functional portion of the MHC molecule" refers to the ability of the fragment to bind a peptide. An effective amount of a refolding agent is that amount necessary for the MHC molecule or chain to regain the capacity to bind an antigen. Therefore, the effective amount will vary depending upon the concentrations of any added antigens and MHC molecules or chains, the reaction conditions, the refolding agent, etc. Reaction conditions will vary widely and one of ordinary skill in the art will be aware of them. In part, the binding of an antigen with an MHC molecule or chain is due to the kinetics and thermodynamics of antigen-MHC molecule reaction, and ways to increase the chances of antigens and MHC molecules colliding are known in the art. A refolding agent can be any chemical or condition, or combination thereof, that produces a functional MHC molecule or chain. Such refolding produces MHC chains with native structure and can occur by forming intrachain disulfide bonds through oxidation or, if such bonds have not been disturbed, by causing the secondary structure of the MHC molecule to reform. Examples of refolding agents include 0 2 , K 3 Fe(CN) 6 , HgCl 2 , H 2 0 2 , quinones, Br , TIAc , MeSOF 2 , NO, and N0 2 . Preferred compositions are 0 2„, K3,Fe(CN)© £ , and HgCl2,, with the most preferred being 0 2 introduced by dialysis in an air-saturated buffer for approximately 12-18 hours. For a guide to the folding and association of proteins in general, see Jaenicke, R. , Prog. Biophys. molec. Biol. 49:117-237 (1987) (incorporated herein by reference for all purposes) . As with refolding, the flexibility of MHC chains also plays a major role in the reassembly of separate α- and / 3-chains to a class II a/β heterodimer. Such reassembly occurs simultaneously with refolding. As shown in the following Examples, chains did not reassemble when they were in the folded state. It is known that several proteins do not reassemble to homo- or heterodimeric conformations when the subunits are folded, but may be reassembled when they are refolded simultaneously, see Huber, R. , Angew. Chem. Internatl. Edit. 27:79-88 (1988) (incorporated herein by reference for all purposes) . similar to the reassembly of heavy and light chains of immunoglobulin Fab fragments, refolding and concomitant reassembly resulted in the following Examples when unfolded chains were used, see Bjόrk, I., et al., Biochemistry, 10:1289-1295 (1971) (incorporated herein by reference for all purposes) . High flexibility of the chains may be a factor in successful reassembly. Figuratively, the two chains may refold and synchronously reassemble like a zipper. Once the heterodimer is formed, it is very stable because the chains are hooked into each other. This picture, although not limiting to the present invention, is consistent with the high stability of MHC molecules observed experimentally by the present inventors. Such rigid chains hooked into each other may act as a trap for bound antigens, which may explain the slow off-rates seen in MHC molecules. Fig. 2 demonstrates the unfolding of separate α- and /-"-chains by use of a western blot. The present invention also comprises processes for adding a desired specific antigen to a functional MHC molecule or chain thereof, produced as described above. These processes are useful in therapeutic administrations and diagnostics wherein a specific treatment or assay is envisioned. The desired specific antigen is added to the MHC molecules or chains thereof that are substantially free of undesired antigens. By specific, it is meant an antigen that can bind with an MHC molecule or chain thereof and that the MHC-antigen complex can then bind with a T lymphocyte receptor. An example of a desired specific antigen includes, but is not limited to, peptide fragments. Also included in this invention are processes for separating an antigen from a MHC molecule or chain thereof, other than by lowering pH, comprising treating the molecule or chain with an effective amount of an unfolding agent. The unfolding agents and their effective amounts are defined above. An unfolding agent that is a reducing agent is preferred, with DTT being most preferred. In these processes, the MHC molecule or chain thereof is not refolded. Such processes have utility in, for example, the storage of antigen-free MHC molecules or chains thereof for later usage. Additionally, the released antigen may be recovered for further identification and analysis. The present invention also comprises compositions produced by the processes discussed above. Compositions comprising MHC molecules or chains thereof that are substantially free of antigens are also included in this invention. Usually, substantially free refers to greater than 70%, with greater than 90% more preferred, and greater than 99% most preferred. These compositions are in a substantially pure form as they are substantially free of antigens. Such purity can be important for therapeutical and analytical purposes. The MHC molecule or chain thereof can be a class I or class II molecule or any of the chain components or a mixture thereof. Additionally, the MHC molecule can be in a floppy (F) or compact (C) conformation. A composition comprising an MHC molecule or chain thereof that is substantially free of antigens is a preferred reagent for screening purposes, such as screening for new therapeutic agents. Thus, an MHC molecule or chain thereof whose binding site is substantially free of antigenic peptides can be used to screen for peptides or other substances that bind with the MHC molecule or chain thereof. Furthermore, an MHC molecule or chain to which a specific desired antigen is bound can be used to screen for potential therapeutic agents that displace the specific antigens. In all such screening procedures, pure MHC molecules or chains are preferred, both from the point of specificity as well as sensitivity. By introducing a labelled (tagged), e.g., radioactive or fluorescent, peptide fragment to MHC molecules or chains thereof that are substantially free of antigens, specific T lymphocyte clones can be assayed in accordance with standard procedures. The T lymphocyte receptor and MHC-peptide complex will bind if specificity exists, and the resultant labelled cell can be detected by methods known in the art. A composition comprising an MHC molecule or chain thereof that is substantially free of undesired antigens to which a desired specific antigen, such as a peptide, is bound, can be used as a therapeutic composition when combined with a pharmaceutically acceptable carrier. Such carriers are known in the art and may be any compatible, nontoxic substance suitable for the delivery of MHC molecules to the patient. Sterile water, alcohol, fats, waxes, and inert solids may be included, as well as adjuvants, if desired. The concentration of MHC molecule in these formulations can vary widely, depending upon the condition to be treated, and the patient's medical history. Such compositions can be employed to treat a patient with an autoimmune condition, wherein the MHC-antigen complex binds to T lymphocyte receptors and inhibits or tolerates. For a specific example of such a disease, see, e.g.. Kumar, V. , et al., Proc. Natl. Acad. Sci. USA 87:1337-1341 (1990) (incorporated herein by reference for all purposes) . A therapeutically effective amount of a present composition is administered to the patient, preferably parenterally, that is, subcutaneously, intramuscularly, or intravenously. Actual methods for preparing administrable compositions, depending upon the made of administration, are readily available to those skilled in the art and are described in more detail in, for example. Remington's Pharmaceutical Science. Mack Publishing, Easton, Pennsylvania (15 ed. 1980) (incorporated herein by reference for all purposes) . Additionally, compositions of the present invention can be conjugated with a cytotoxic agent and a pharmaceutically acceptable carrier, the latter of which is discussed above. Cytotoxic agents are known in the art and include, but are not limited to, various radioisotopes, the alkaloid vindesine, the ricin A-chain, daunomycin, and the like. Such agents can also include antimicrobial substances, including antiviral compositions, directed at a specific antigenic source. Antibodies can be prepared by injecting MHC molecules or chains thereof into a foreign host, in accordance with conventional techniques. Alternatively, monoclonal antibodies may be prepared, following the technique elaborated in Kohler & Milstein, Nature 256:495 (1975). Of particular interest to the present invention is the monoclonal antibody, 28-18-85, see Ozato, K. , et al., J. Immunol. 126:317-321 (1979) (incorporated herein by reference for all purposes) , which recognizes an epitope expressed only by a/β heterodimers, as shown in Fig 4. Once the antibodies are having suitable specificity have been produced, a wide variety of immunological assay methods may be utilized to ascertain the presence of such MHC molecules or chains thereof. Numerous competitive and non-competitive protein binding assays have been described in the literature, and a large number are commercially available. Typically, the assays entail the use of labels on antibodies and these include radionuclides, fluorescent tags, enzymes, enzyme substrates, enzyme cofactors, enzyme inhibitors, etc. In the Examples below, western blotting using an alkaline phosphatase conjugated goat anti-mouse antibody was used for detection. Thus, a variety of therapeutic and screening utilities can be envisioned by one skilled in the art using the compositions of MHC molecules and/or chains thereof of the present invention, alone or in combination with other compositions and agents. It is to be understood that the above description and the following experimental section are intended to be illustrative and not restrictive. Many variations and applications will be readily apparent to one of ordinary skill in the art upon reviewing this disclosure. Although slight modifications of experimental details may have to be introduced for haplotypes other than murine IA d class II MHC molecules, the present processes provide a means to overcome the well-known problems of antigens, including autologous peptides, occupying the binding site. Examples Example 1: Removal and replacement of antigens from a class II MHC molecule, Murine IA d Sample preparation Murine IA d , which is a class II MHC molecule, was affinity purified from A20.1.11 cells (available from American Type Culture Collection as deposit no. TIB 208) using the antibody MK-D6 (available from American Type Culture Collection as deposit no. HB 3), see Braunstein, N.S., et al., Proc. Natl. Acad. Sci. USA 84:2921-2925 (1987) (incorporated herein by reference for all purposes) , as described in Rothenhausler, B. , et al., Proc. Natl. Acad. Sci. USA 87:352-354 (1990) (incorporated herein by reference for all purposes) . 30 mM octyl-/9-D-glucopyranoside was replaced by 2 mM dodecyl-3-D maltoside (DM) (Calbiochem, La Jolla, California) , which has a lower critical micellar concentration. The IA d -containing fractions were pooled and dialyzed in 10 mM Tris/HCl, pH 8.3, 150 mM NaCl, 0.02% NaN 3 , 2 mM DM over night. This preparation, which is referred to as IA d , contained the compact (C) and floppy conformations (F) and separate α- and ,9-chains of class II MHC molecules. To isolate the α- and 3-chains separately, purified IA d was run on preparative SDS gels, omitting ,9-mercaptoethanol, and not boiling the samples. The α- and ,9-chains were electroeluted, see Hunkapiller, M.W. , Methods in Enzymology 91:227-236 (1983) (incorporated herein by reference for all purposes) , from these gels in 25 mM Tris, 0.2 M glycine, pH 8.2, 0.1% SDS. Reduction a d Reoxiflatiop reactions IA d (30 μg/ml) was reduced with 1 mM and 10 mM DTT in 10 mM Tris/HCl pH 8.3, 150 mM NaCl, 0.02% NaN 3 , 2 mM DM by incubation for 1 h at 37 "C. The α- and jS-chains (20 μg/ml each) were incubated for 1 h at 37 'C in 1 mM or 10 mH DTT in 25 mM Tris, 0.2 M glycine, pH 8.2, 0.1% SDS either separately or after mixing equal amounts. For reoxidation, the reduced samples were dialyzed in the at least 1000 fold volume of 10 mM Tris/HCl pH 8.3, 150 mM NaCl, 0.02% NaN 3 , 2 mM DM for 18 h. 9DC DiaCell™ Dialysis Capsules equipped with Spectrapor dialysis tubings with a molecular weight cutoff of 12 kD were used. The sample volume was typically 50 μl. A basin with a large enough surface to allow for oxygen saturation of the buffer was used. Peptide incubation and analytical gel electrophoresis IA d or the α- and 3-chains were incubated with 100 μM FOva(323-339) for 1 h at 37 "C before reduction or after reoxidation. Reduction was performed after incubation with lOOμM FOva(323-339) . After the incubations, the samples were 33 Table I Relative Protein Concentrations (absorbance x mm) F C α β Relative Fluorescence Intensities (cps x 10 ~3 x channels) β 32 loaded immediately on 12.5% SDS gels. Prior to fixation and silver staining, the gels were scanned for fluorescent peptides on a fluorescence microscope as described in Rothenhausler, B. , et al., Proc. Natl. Acad. Sci. USA 83:352-354 (1990) (incorporated herein by reference for all purposes) . The gels were silver-stained, see Heukeshoven, J., et al., Electrophoresis 6:103-112 (1985) (incorporated herein by reference for all purposes) , and relative protein concentrations determined by scanning the silver-stained gels on a LKB UltroScan XL Laser Scanner. The scans for fluorescent peptides and for silver-stained proteins were corrected for background. Western blotting Proteins were transferred from SDS gels to nitrocellulose membranes overnight at 30 V in 20 mM Tris, 150 mM glycine, pH 8.0. MHC heterodimers were detected with the monoclonal antibody 28-16-8S (available from American Type Culture Collection as deposit no. HB 35), see Ozato, K., et al., J. Immunol. 126:317-321 (1979) (incorporated herein by reference for all purposes) . An alkaline phosphatase conjugated goat anti-mouse antibody (Sigma, St. Louis, Missouri) was used for detection, and 0.1 mg/ml nitro blue tetrazolium and 0.05 mg/ml 5-bromo-4-chloro-indolylphosphate in 50 mM sodium carbonate buffer, pH 9.5, 2 mM MgS0 4 was used for staining. Results Fig. 1 shows the effect of reduction with 1 mM and 10 mM DTT and reoxidation on IA d . Fig. 1A shows the silver-stained SDS gels and Fig. IB shows the corresponding scans for fluorescent peptides. The relative distribution of the two a/β heterodimeric conformations, compact (C) and floppy (F) , and the separate α- and ,9-chains as revealed by scanning the silver-stained gel on a gel scanner, and the relative amount of peptide bound to each of these conformations are given in Table I below. In Table I, the relative protein concentrations and fluorescence intensities of the samples of Figs. 1A and B are given. The relative protein concentrations were determined by scanning the silver-stained gel on a gel scanner. The numbers were obtained by integrating the area under the peaks. Relative fluorescence intensities were determined by scanning the gel on a fluorescence microscope prior to fixation. The numbers were obtained by multiplying the peak maximum with the half width of peak. FOva(323-339) was smeared over the gel. The fluorescence background increased with the length of the gel. Therefore, the detection limit for the ,3-chain is higher than for the α-chain, which again is higher than for floppy (F) conformation. IA d was incubated with 100 μM FOva(323-339) for 1 h at 37 "C (Figs. 1A and B, Lane 1) . Scanning of the silver-stained gel on a gel scanner revealed a relative distribution of floppy : compact : α chain : β chain = 1.23 : 2.19 : 0.15 : 0.13 (Table I). As shown in Fig. IB, Lane 1, and reported in Rothenhausler, B. , et al., Proc. Natl. Acad. Sci. USA 83:352-354 (1990) (incorporated herein by reference for all purposes), all of these conformations bind specific peptides. However, the relative concentrations of separate chains in the present preparation was particularly low. Reduction of the disulfide bonds with 1 mM and 10 mM DTT led, in both experiments, to the complete loss of peptides bound to IA d (Fig. IB, Lanes 2 and 4, respectively and Table I). Only a small amount of peptide remained bound to the compact conformation after reduction with 1 mM DTT, which completely disappeared upon reduction with 10 mM DTT. This indicates that compact (C) is the most stable conformation. The effect of reduction on the secondary structure can be seen in the silver-stained gel: whereas the compact (C) conformation is unaltered, floppy (F) conformation disappeared almost completely (Fig. 1A, Lanes 2 and 4) . Most of floppy (F) conformation disassembled to separate chains (Table I) . Both the separate α- and ,9-chain migrated at positions corresponding to higher apparent molecular weights, indicating unfolding of the proteins. Reoxidation by dialysis in air saturated buffer restored not only the floppy (F) conformation from the separate chains (Table I) and the folded conformation of both the α- and the β-chain (Fig. 1A, Lanes 3 and 5) . In addition, peptide binding was restored (Fig. IB, Lanes 3 and 5 and Table I) . Importantly, the amount of peptide bound to each of these conformations was significantly higher (Table I) . The fluorescence associated with floppy (F) conformation rose from 4,900 cps x channels of the untreated sample to 19,800 cps x channels and 16,000 cps x channels of the reduced and reoxidized samples, respectively. Although less pronounced, a significant increase of peptide binding to the compact (C) conformation and to the separate α- and >9-chains was observed. These results are interpreted, but do not limit the present invention, as being related to the release of previously bound peptides by reduction and removal by dialysis. Reoxidation and refolding yields MHC molecules lacking bound peptides. Therefore more binding sites are accessible for FOva(323-339) . With the particular preparation used herein, it was found that the peptide binding of all conformations was increased by a factor of greater than 2.5 after reduction with 10 mM DTT and reoxidation (see Table I) . However, other preparations showed peptide binding capacities to be increased by a factor of ten or more. in order to investigate the conformational changes and enhanced peptide binding capacity of IA d after reduction and reoxidation in more detail, the separate α- and ,3-chains were isolated by electroelution from preparative SDS gels. As shown in Fig. 2, the ,9-chain migrated with J ~ ~ ~w = 27.5 kD (Lane 1). After reduction with 1 mM DTT (Lane 2), 35% of this chain migrated at MW app = 28.5 kD and 25% with MW* pp = 30 kD. After reduction with 10 mM DTT, 100% of the ,3-chain migrated at MW app = 27.5 kD (Lane 3). Again, boiling did not alter the apparent molecular weight (Lane 4) . As also shown in Fig. 2, the α-chain migrated with MW app = 33 kD (Lane 6) . After reduction with 1 mM DTT for 1 h at 37 *C, 75% of this chain migrated at MW* PP = 34.5 kD (Lane 7) . After reduction with 10 mM DTT, 100% of the α-chain migrated at MW ap = 34.5 kD (Lane 8). Boiling for 30 min in 2% SDS in the presence of 10 mM DTT did not alter the apparent molecular weight (Lane 9) . Thus, both chain migrated with higher apparent molecular weights after reduction. This is interpreted, but not limiting to the present invention, as due to an increased effective volume of the proteins, i.e., to a partial or complete unfolding of their secondary structure. As the unfolding of the ,3-chain occurs in a two-step reaction, it is assumed that one of the disulfide bonds of the ,3-chain has a higher redox potential than that of the other bond. Boiling in SDS in the presence of high concentrations of DTT leads to a complete unfolding of the secondary structure. Therefore, it is postulated that, upon incubation in 10 mM DTT for 1 h at 37 "C, IA d not only disassembles into separate chains but those chains also unfold, possibly even to a random conformation. This interpretation, although not limiting to the present invention, easily explains the fact that reduced IA d does not bind peptides (Fig. 1) . In this experiment, the compact (C) conformation was found to be somewhat resistant to reduction. This might be explained, but does not limit the present invention, by assuming a higher redox potential of one or more of the disulfide bonds in this a/β heterodimeric conformation than in the floppy (F) conformation or in the separate chains. Alternatively, the disulfide bonds might be reduced, but the noncovalent interaction between the chains is stronger than in the floppy (F) conformation, thus preventing disassembly and unfolding. Another possibility is that the disulfide bonds might be reduced, but the noncovalent interaction between the chains is stronger than in the floppy (F) conformation, thus preventing disassembly and unfolding. Boiling in SDS in the presence of DTT or mercaptoethanol does cause the compact (C) conformation to disassemble and unfold. The separate chains can be reassembled to the a/β heterodimer. Fig. 3A shows the electroeluted α- and 3-chains in Lanes 2 and 3. A small fraction of both the α- and /3-chains dimerized, migrating with M 81 * = 68 kD and 56.5 kD, respectively. This is analogous to the dimerization of immunoglobulin heavy and light chains, see Bjόrk, I., et al.. Biochemistry 10:1289-1295 (1971). The chains were mixed and incubated at 37 "C with and without peptide for 1 h and 18 h (Lanes 4-7) . Under these conditions, reassembly was not achieved. The mixed, separate chains were reduced with 1 and 10 mM DTT for 1 h at 37 "C (Lane 8 and 9, respectively), thus unfolding the proteins. Reoxidation by dialysis in air saturated buffer (Lanes 10-13) not only refolded the separate chains as evidenced from their lower apparent molecular weights, but also yielded considerable amounts of a/β heterodimers migrating with MW app 64 kD. This corresponded to the floppy (F) conformation and was distinctly different from the α- and ,9-chain dimers discussed above. The yields of reassembled a/β heterodimer were 8% after reduction with 1 mM DTT and reoxidation (Lane 10) , 18% after reduction with 10 mM DTT and reoxidation (Lane 11) , 5% after reduction with 1 mM DTT and incubation with 100 μM FOva(323-339) following the reoxidation (Lane 12), and 17% after reduction with 10 mM DTT and incubation with 100 μM FOva(323-339) following the reoxidation (Lane 13) . Thus, the yield was significantly higher when the proteins were first unfolded completely, indicating that reassembly occurs simultaneously with refolding. The incubation with peptides had no pronounced influence on the reassembly efficiency. Figs. 3B and 3C show scans for proteins and for fluorescent peptides of Lanes 6, 12, and 13 of Fig. 3A. The positions of separate chains, homodimers, and the compact (C) and floppy (F) heterodimers are indicated. Fig. 3B provides clear evidence that reassembly only occurs if the separate chains have been previously unfolded. In Lane 6, folded separate chains were incubated. In Lanes 12 and 13, the chains were first unfolded and reassembled during refolding to yield predominantly floppy (f) and, to some extent, compact (C) conformations. It has been shown that MHC molecules disassemble in a consecutive reaction: compact —> floppy —> disassembled chains. The results presented herein indicate, but do not limit the present invention, that the separate chains first reassemble to yield floppy (F) conformation, then the reaction proceeds to yield compact (C) conformation. Therefore, it is the reverse reaction of the disassembly reaction. At molecular weights higher than those of floppy (F) conformation, aggregated chains can be seen. However, comparison with Fig. 3C shows that those aggregates do not bind peptides, and thus may be aggregated in an unfolded state. As the concentration of free .-chain is decreased compared to the α-chain, it is assumed that the high molecular weight aggregates are ,9-chain oligomers. The samples of Lanes 6, 12, and 13 were incubated with FOva(323-339) under identical conditions directly before applying them to the gel. After normalizing the protein concentrations using the scans for proteins shown in Fig. 3B, which was necessary because the overall recovery was not quantitative and the ,9-chains oligomerized, the peptide binding capacity of the α- and ,9-chains increased by factors of 11 and 9, respectively, in the particular preparation used herein. To obtain additional evidence that the product of the reassembly experiment is the heterodimeric floppy conformation (F) , the monoclonal antibody 28-16-8S, see Ozato, K. , et al., J. Immunol. 126:317-321 (1979) (incorporated herein by reference for all purposes) , was used in western blot experiments. Fig. 4 shows a silver-stained gel of an untreated IA d preparation (Lane 1) . The same preparation was blotted and stained using 28-16-8S (Lane 2) . Comparison of the silver-stained gel and the blot of the untreated sample, Lane 1 with Lane 2, respectively, shows that 28-16-8S recognizes the compact (C) and floppy (F) conformation but not the separate chains, although the relative concentrations of α chain : ,9 chain : compact: floppy were about 1:1.12:1.25:1.37. A western blot of a refolded and reassembled sample is shown in Lane 3. Bands at the positions expected for floppy (F) and compact (C) conformations are seen. This result provides evidence, but does not limit the present invention, that the experiments described herein yield the heterodimeric floppy (F) and compact (C) conformations from separate, unfolded chains. It is noted that the reduction and reoxidation experiments described above quantitatively release antigenic peptides known to co-purify with MHC molecules, see Demotz, S., et al.. Nature, 243:682-684 (1989); Wallny, H.J. , et al., Nature 343:275-278 (1990) (these publications are incorporated herein by reference for all purposes) . As shown in Fig. 1, reassembly is not essential for this purpose. Table I shows that the binding capacity for antigenic peptides is greatly increased after reduction and reoxidation. Although not quantitatively determined, the stoichiometry of antigenic peptide binding to MHC molecules resulting from processes of the present invention may be close to 100%. Example 2: Removal and replacement of antigens from a class I MHC molecule, HLA-A2 MHC Purification HLA-A2, which is a class I MHC molecule, was purified, with minor modifications from 721.53 cells, as described in Chen et al. Nature 337:743-745, which is incorporated herein by reference for all purposes. The cells were lysed at a concentration of 10 7 cells per ml in 0.75% Nonidet P-40/10 mM Tris-HCl, pH 8.3/150 mM NaCl/0.02% NaN_ 10μM phenylmethylsulfonyl fluoride aprotinin (10 μg/ml)/0.01% thimerosal by stirring on a vortex mixer. A pool of glycoproteins was obtained by chromatography on lentil lectin-Sepharose. The eluate from the Sepharose column was applied to a column with covalently linked monoclonal antibody from the CRll.351 hybridoma. CR11.351 antibody was purified by chromatography on protein G-Sepharose (Calbiochem) . The antibody column was washed with 20 column volumes of 2 mM dodecyl ,9-D-maltoside (DM) (Calbiochem)/10 mM Tris-HCl, pH 8.3/150 mM NaCl/0.02% NaN and the antibody was eluted with 2 mM DM/50 Na 2 C0 3 /500 mM NaCl and neutralized immediately. If necessary, samples were concentrated by exposing them in a dialysis bag to solid polyethylene glycol. Samples were then dialyzed in 2 mM DM/10 mM Tris-HCl, pH 8.3/150 mM NaCl/0.02% NaN 3 . The heavy chain and β -microglobulin were detected by nondenaturing SDS/PAGE, although they were found to be disassembled, presumably due to the presence of SDS. For some experiments, the heavy chain was isolated by electroelution from a non-denaturing SDS gel. Peptide Incubation and Detection Typically 30 μl of a 8.0 μg/ml solution of HLA-A2 was incubated in 2 mM DM/10 mM Tris-HCl/150 mM NaCl/0.02% NaN 3 for 1 hr at 37 ~ C with 10 μM MHC antigens. Such antigens include the N-terminally fluoresceinated FLU-MI peptide (F-FLU-Ml) , an N-terminally fluoresceinated peptide representing amino acids 88-104 of chicken ovalbumin, and N-terminally fluoresceinated Gly-(Ala) 4 . In the experiments where the eletroeluted heavy chain was used, peptide incubation was in the electroelution buffer (see above) . The samples used for reduction and reoxidation experiments were incubated for 1 hr at 37 "C or 1 hr at 90 "C in 2 mM DM/10 mM Tris-HCl/150 mM NaCl/0.02% NaN 3 containing 10 mM dithiothreitol (DTT) . Some samples contained an additional 2% (weight/volume) SDS. Samples that were to be oxidized and not to be oxidized were treated differently. The samples not reoxidized were incubated for 1 hr at 37 • C with 10 μM F-FLU-Ml in the DTT containing buffer of their previous incubation. Reoxidation was performed by dialysis in air-saturated 2 mM DM/10 mM Tris-HCl/150 mM NaCl/ 0.02% NaN 3 for 18 hr at 37 ' C. 9DC DiaCell capsules (InstuMed, Union Bridge, MD) , equipped with Spectrapor tubings with a molecular size cutoff of 12 kD were used. The reoxidized samples were then incubated for 1 hr at 37 β C with premixed lOμM F-FLu-Ml and FLu-Ml at 10 μM or at 100 μM. All samples were loaded on a SDS gel (omitting boiling and reduction) , and the gel was scanned on a fluorescent microscope as described Dornmair et al. Cold Spring Harbor Symp. Quant. Biol. 54:409-416 and Rothenhausler et al. Proc. Natl. Acad. Sci. USA 83:352-354. The gel then was fixed and stained with silver. Peptide Synthesis The amide derivative of human influenza matrix protein fragment FLU-MI, H 2 N-YTKGILGFVFT-LTV-NH 2 , was synthesized with a tyrosine residue added at the N terminus, by the solid-phase technique on a Milligen model 9050 peptide synthesizer. PepSyn KB resin for synthesis of peptide amides and the pentafluorophenyl esters of side-chain- and N α -fluorenylmethoxycarbonyl-protected amino acids were used in the synthesis. After removal of side-chain protecting groups and cleavage from the resin with trifluoroacetic acid and carbocation scavenger, the crude peptide was isolated, dried, and redissolved in 0.1% trifluoroacetic acid. After purification by reverse-phase HPLC, the purified lyophilized peptide was dissolved in dimethyl sulfoxide and labeled with fluoresceinated peptide was precipitated from the reaction mixture with 15 volumes of ethyl acetate. The precipitant was washed with ethyl acetate, dried, and redissolved in 0.1% trifluoroacetic acid in 70% (volume/volume) aqueous acetonitrile as buffer B and 0.1% trifluoroacetic acid as buffer A. RESULTS Fig. 5A shows a silver-stained SDS gel of affinity-purified HLA-A2 that has been incubated with the purified fluorescein-labeled peptide F-FLU-Ml prior to applying the sample to the gel, omitting prior boiling and reduction. The heavy chain migrates with an apparent molecular mass of 43 kD. An additional band is identified that migrates with an apparent molecular mass of 88 kD. This might be the dimer of the heavy chain. Fig. 5B shows a scan of this gel for fluorescent peptides. Although our technique does not determine the stoichiometry of peptide to protein, a strong fluorescent signal due to F-FLU-Ml is seen to be associated with the heavy chain. F-FLU-Ml also binds to the band at 88 kD. The binding of the antigen, F-FLU-Ml, is specific. Fig. 5C shows an experiment identical to that in Fig. 5B, except that F-FLU-Ml was replaced by a fluoresceinated peptide representing amino acids 88-104 of chicken ovalbumin. No binding could be detected. Furthermore, no detectable binding of the N-terminally fluoresceinated peptide Gly-(Ala) 4 could be measured. To investigate whether β -microglobulin is necessary for binding of F-FLU-Ml to HLA-A2, the heavy chain of HLA-A2 from a preparative SDS gel was electroeluted and subsequently incubated with F-FLU-Ml. Fig. 5D demonstrates that F-FLU-Ml binds to the heavy chain in the absence of 0 2 -microglobulin. Thus, there is a direct on-reaction of F-FLU-Ml to the heavy chain of HLA-A2 in the absence of ,3 2 -microglobulin. Figs. 5E-H shows a SDS gel of HLA-A2 reduced with lOmM DTT and scans of the gel for fluorescent peptides under various conditions. All samples were reduced for 1 hr with 10 mM DTT in buffer containing 2 mM DM. The samples of Fig. 5H contained in additional 2% SDS. Incubation was for 1 hr at 37 • C (Fig. 5F and 5H) or at 90 • C (Fig. 5G) . Upon reduction, the apparent molecular mass shifted to 45 kD, but no difference could be detected between incubation in DM at 37 *C or at 90 # C or in SDS. The heavy chain of HLA-A2 still bound fluorescent peptide in the presence of DM and 10 mM DTT (Fig. 5F and 5G) . The peptide binding capacity was even enhanced when the incubation was at 90 *C (Fig. 5G) . However, addition of 2% SDS prevented peptide binding (Fig. 5H) . Therefore, despite identical reducing conditions, the type of detergent (compare Figs. 5F and 5H) , and the temperature (compare Figs. 5F and 5G) are important. Empirically, elevated temperature during reduction promoted peptide binding to the reduced heavy chain; the presence of SDS prevented it. Oxidation of the reduced sample (Fig. 5F) by dialysis in air-saturated buffer shifts the apparent molecular mass back to 43 kD (Fig. 5J) , indicating refolding. However, the yield of refolded protein was only 20%. Even with less protein there was about the same amount of F-FLU-Ml associated with the band corresponding to the heavy chain of HLA-A2 as was observed prior to reduction (compare Fig. 5B and 5J) . Therefore, the peptide binding capacity is enhanced by a factor of five in this preparation. This factor varied with different preparations and ranged from three to ten. We assume that during the first hours of dialysis, when DTT is still present, bound peptides dissociate with fast off-rates and are removed by dialysis. The putative fast peptide-off rates are due to reduced disulfide bonds and the presence of detergent which unfold the chain sufficiently to allow dissociation. Binding of the F-FLU-Ml peptide to the refolded heavy chain is specific, because the fluoresceinated peptide may be displaced by unlabeled FLU-MI (Fig. 5K and 5L) . The displacement of F-FLU-Ml is stoichio etric as judged by comparing the peak intensities and the molar relations. This is further independent evidence that the peptide moiety of F-FLU-Ml binds specifically to the binding site of the heavy chain of HLA-A2. It proves in particular that the refolded molecules also regain their specificity. The example shows that the purified heavy chain of HLA-A2 is able to bind peptide in the absence of /92-microglobulin. Therefore, binding of the antigen to the purified heavy chain prior to assembly with /32-microglobulin is possible. Additionally, this example shows that antigen binds to already folded and refolded heavy chain of a class I MHC molecule , as well as to an heavy chain unfolded in a unfolding agent known to disturb at least the tertiary structure, thus enhancing the flexibility and dynamics of the MHC molecule without completely unfolding the MHC molecule. It is to be understood that the above description and examples are intended to be illustrative and not restrictive. Many variations and applications will be readily apparent to one of ordinary skill in the art upon reviewing this disclosure.

Claims: WHAT IS CLAIMED IS: 1. A method for removing an antigen bound to an antigen-binding fragment of a major histocompatibility complex (MHC) molecule comprising the steps of: unfolding the antigen-binding fragment of the MHC molecule by treating the antigen-binding fragment with an unfolding agent to release the antigen; separating the released antigen from the unfolded antigen-binding fragment of the MHC molecule; and refolding the unfolded antigen-binding fragment of the MHC molecule with a refolding agent to produce a functional antigen-free antigen-binding fragment of the MHC molecule. 2. The method of claim 1 further comprising the step of removing said unfolding agent from the unfolded antigen-binding fragment of the MHC molecule after the step of separating the released antigen and before the step of refolding the unfolded fragment. 3. The method of claim 1 wherein the antigen- binding fragment of the MHC molecule comprises an MHC molecule. 4. The method of claim 3 wherein the antigen- binding fragment of the MHC molecule is an MHC molecule selected from the group consisting of class I MHC molecules and class II MHC molecules. 5. The method of claim 1 wherein the antigen- binding fragment of the MHC molecule comprises a binding chain of an MHC molecule. 6. The method of claim 5 wherein the antigen- binding fragment of the MHC molecule is selected from the group consisting of α-chains from a class I MHC molecule, α-chains from a class II MHC molecule, ,9-chains from a class I MHC molecule, and mixtures thereof. 7. The method of claim 1 wherein the antigen- binding fragment of the MHC molecule is a class II MHC molecule in a floppy conformation or a compact conformation. 8. The method of claim 1 wherein the unfolding agent is a reducing agent. 9. The method of claim 8 wherein the reducing agent is a thiol-containing compound. 10. The method of claim 9 wherein the reducing agent is selected from the group consisting of dithiothreitol (DTT) , dithioerythritol, and mercaptoethanol. 11. The method of claim 10 wherein the reducing agent is dithiothreitol (DTT) . 12. The method of claim 11 wherein, during the step of unfolding, the antigen-binding fragment of the MHC molecule is treated with DTT having a concentration in the rage from about 1 mM to about 10 mM. 13. The method of claim 11 wherein the step of unfolding comprises incubating the antigen-binding fragment of the MHC molecule and DTT for a time period of approximately one hour at a temperature of about 37 "C. 14. The method of claim 8 wherein the refolding agent is an oxidant. 15. The method of claim 14 wherein the refolding agent is selected from the group consisting of 0 2. K3„Fe(CN)6. and HgCl . 16. The method of claim 8 wherein the step of refolding comprises dialyzing the pocket of the MHC molecule in air-saturated buffer, whereby the unfolded pocket treated with 0_. 17. The method of claim 1 further wherein the step of refolding comprises treating the unfolded antigen-binding fragment with a refolding agent and a second antigen, wherein the functional antigen-binding fragment of the MHC molecule has said second antigen bound thereto. 18. The method of claim 1 wherein the functional antigen-free antigen-binding fragment of the MHC molecule is substantially free of bound antigen. 19. The method of claim 18 further comprising the step of incubating the functional antigen-binding fragment of the MHC molecule with a second antigen to produce a second antigen-binding fragment of the MHC molecule having a second antigen bound thereto. 20. The method of claim 1 wherein the antigen is an autologous peptide. 21. A method for removing an antigen bound to an antigen-binding fragment of a major histocompatibility complex (MHC) molecule comprising the steps of: unfolding the antigen-binding fragment of the MHC molecule by treating the antigen-binding fragment with an unfolding agent to release the antigen; separating the released antigen from the unfolded antigen-binding fragment of the MHC molecule; refolding the unfolded antigen-binding fragment of the MHC molecule into a refolded functional antigen-free antigen-binding fragment of the MHC molecule by treating the unfolded antigen-binding fragment with a refolding agent; and binding a desired antigen to the functional antigen- binding fragment of the MHC molecule by treating the refolded functional fragment of the MHC molecule with said desired antigen. 22. A method for separating an antigen from an antigen-binding fragment of a major histocompatibility complex (MHC) molecule in solution comprising treating the pocket of the MHC molecule with an unfolding agent while maintaining the solution at a constant, non-acidic, pH. 23. A substantially pure composition comprising an antigen-binding fragment of a major histocompatibility complex (MHC) molecule that is substantially free of bound antigen, said MHC molecule being capable of binding to an antigen. 24. A substantially pure composition comprising the composition of claim 22 bound with a desired antigen. 25. The substantially pure composition of claim 23 wherein the antigen-binding fragment of the MHC molecule comprises an MHC molecule selected from the group consisting of class I MHC molecules and class II MHC molecules. 26. The method of claim 23 wherein the antigen- binding fragment of the MHC molecule comprises a binding chain of a MHC molecule selected from the group consisting of α-chains a class I MHC molecule, α-chains a class II MHC molecule, β-chains from a class I MHC molecule, and mixtures thereof. 27. A composition comprising an antigen-binding fragment of a major histocompatibility complex (MHC) molecule produced by the method comprising the steps of: unfolding the antigen-binding fragment of the MHC molecule by treating the antigen-binding fragment with an unfolding agent to release the antigen; separating the released antigen from the unfolded antigen-binding fragment of the MHC molecule; and refolding the unfolded antigen-binding fragment of the MHC molecule with a refolding agent to produce a functional antigen-free antigen-binding fragment of the MHC molecule. 28. A composition comprising an antigen-binding fragment of a major histocompatibility complex (MHC) molecule produced by the method comprising the steps of: unfolding the antigen-binding fragment of the MHC molecule by treating the antigen-binding fragment with an unfolding agent to release the antigen; separating the released antigen from the unfolded antigen-binding fragment of the MHC molecule; refolding the unfolded antigen-binding fragment of the MHC molecule into a refolded functional pocket of the MHC molecule by treating the unfolded antigen-binding fragment with a refolding agent; and binding a desired antigen to the functional antigen- binding fragment of the MHC molecule by treating the functional pocket of the MHC molecule with said desired antigen.

Inventors: DORNMAIR KLAUS, MCCONNELL HARDEN M, DORNMAIR, KLAUS, MCCONNELL, HARDEN, M.
Applicants: UNIV LELAND STANFORD JUNIOR, THE BOARD OF TRUSTEES OF THE LELAND STANFORD JR. U
Application year: 1991
Main classification: C07K3
IPC codes: A61K38, C07K1, C07K14