METHOD OF IDENTIFYING MHC-CLASS I RESTRICED ANTIGENS ENDOGENOUSLY PROCESSED BY A CELLULAR SECRETORY PATHWAY

Abstract: A method of identifying antigens which are capable of being endogenously processed by a cellular secretory pathway comprising: introducing an antigen into a donor cell lacking in MHC class I molecules, incubating in an in vitro medium the donor cells, primed cytotoxic T lymphocytes having specificity for the antigen, and target cells which express MHC class I molecules and are labelled with a detectable intracellular marker, under suitable conditions such that the donor cells remain intact, and measuring the amount of detectable marker released into the incubation medium. A method of assaying a medium for the presence of a substance that affects processing of an endogenously processed antigen in a cellular secretory pathway; and a method of characterizing a tumor or viral antigen capable of being endogenously processed by a cellular secretory pathway.

Description: Title: METHOD OF IDENTIFYING MHC-CLASS I RESTRICTED ANTIGENS ENDOGENOUSLY PROCESSED BY A CELLULAR SECRETORY PATHWAY FIELD OF THE INVENTION The mvention relates generally to a method of identifying antigens which are capable of being endogenously processed by a cellular secretory pathway and to a method of assaying a medium for the presence of a substance that affects processing of an endogenously processed antigen in the cellular secretory pathway Further, the invention relates to a method of characterizing cytotoxic T cell antigens capable of being endogenously processed by the cellular secretory pathway BACKGROUND OF THE INVENTION The cytotoxic T lymphocyte (CTL) response is a major component of the immune system, active in immune surveillance and destruction of infected or malignant cells and invading organisms expressing foreign antigens on their surface The gand of the antigen-specific T lymphocyte receptor is a complex made up of a peptide fragment of a foreign antigen 8 to 10 amino acids in length, presented in the groove of MHC class I molecules Unlike B cells, T cells do not recognize intact native antigen molecules In general, cytotoxic T cell activation requires that the antigen be processed endogenously and cleaved into specific peptide fragments which are presented on the surface of antigen processing cells in association with class I MHC molecules MHC class I molecules are normally expressed at the cell surface as ternary complexes formed by a heavy chain of 46 kD, a light chain called β 2 -mιcroglobuhn ( β 2 m) of 12 kD and a peptide composed of 8-10 ammo-acids (van Bleek, G M and S G Nathenson, Na t u re 348 213, 1990, Zhang, W et al , Proc Natl Acad S USA 89 8403, 1992, Matsumura, M et al , Science 257 927, 1992, and Latron, F , et al , Science 257 964, 1992) Formation of the ternary complex is thought to involve transport into the lumen of the endoplasmic reticulum (ER) of peptides generated by protein degradation in the cytoplasm (Nuchtern, J G et al , Na ture 339 223, 1989, Yewdell, ] W and J R Bennink, Science 244 1072, 1989, and Cox, J H et al , Science 247715, 1990) The study of mutant cell lines selected for their low expression of MHC class I molecules at the cell surface has provided insights into the molecular events required for antigen processing These studies have allowed the identification of two genes located in the MHC region which encode proteins of the ATP binding cassette (ABC) famih These genes, called TAP-1 and TAP-2, have been implicated in transport of peptides from the cytoplasm to the lumen of the ER (Deverson, E V et al., Nature 348 738, 1990, Trowsdale, J et al , Natiu e 348 741, 1990, Spies, T et al Nature 348 744, 1990, Monaco, J J et al , Science 250 1723, 1990, Spies, T and R DeMars, Nature 351 323, 1991, Bahram, S et al , Proc Natl Acad Sci USA 88 10094,1991, Spies, T et al , Natw e 355 644, 1992, Kelly, A et al , Natuie 355 641 , 1992, Powis, S H et al , Proc Natl Acad Set USA 89 1463, 1992, and Colonna, M et al , Proc Natl Acad Set USA 89 3932, 1992) Two other MHC linked genes, LMP-2 and -7 (Monaco, J J and McDevitt, 1982, Proc Natl Acad Sci USA 79 3001), are components of the proteasome, a cytoplasmic multicatalytic protease complex, which is likely responsible for some aspects of protein degradation for antigen processing (Ortiz-Navarette, V et al , Nature 353 662, 1991, Brown, M G et al , Nature 353 355, 1991, Glynne, R et al , Nature 353 357, 1991, Martinez, C K and J J Monaco, Nature 353 664, 1991, Kelly, A et al , Nature 353 667, 1991, Yang, Y et al , Proc Natl Acad Sci USA 89 4928,1992,Goldberg, A L and K L Rock, Nature 357 375, 1992) CTL recognize MHC class I molecules bearing endogenously processed foreign or aberrant self proteins, thereby enablmg the immune system to respond to intracellular pathogens or tumor transformation The endogenous cellular processing pathways of MHC class 1 antigen presentation is thought to require that the antigen be expressed endogenously in the cytoplasm or ER of the presenting cell The endogenous peptides associate with nascent membrane bound MHC class I molecules before they leave the ER and are eventually presented at the cell surface bound to the MHC class I molecules (Yewdell, W and J R Bennrnk, 1989, Science 244 1072) Peptides unable to bind MHC Class I because they are in excess or lack the correct MHC binding motif for the MHC haplotype are thus far undetectable by the techniques commonly used in the field and are presumed to be short lived and degraded (Falk et al , Nature, 348 248-251, 1990, Rotzschke et al , Nature, 348 252-254, 1990) But recent results (Arnold et al , / txp Med , 182 885-889, 1995) suggest that peptides not able to bind to a MHC class I molecule intracellularly may be found bound to heat shock proteins (HSPs) such as gp96 These authors show that cellular antigens are represented by peptides associated with gρ96 molecules independently of the MHC class I expressed confirming earlier results by Udono and Sπvastava (J Exp Med , 178 1391-1396, 1993, / Immunol , 152 5398-5403, 994) Gp96 extracted from a specific cell are able to induce "cross-priming" Huang et al [Science, 264 96-965, 1994) have elucidated mechanisms in which proteins or peptides may either be exchanged from donor to recipient after release from the cell surface or released following uptake of antigen by the antigen processing donor Though these phenomena may operate under specific circumstances, they relate to the processing of exogenous antigens rather than endogenous antigens Autoimmune disease results from the misdirection of the immune response to self antigens resulting in host tissue damage Autoimmune diseases may be triggered by mfectious agents, such as bacteria and viruses, environmental conditions and certain drugs and tox s Several mechanisms have been put forward which mav account for some, but not all of the features of autoimmunity For example, autoimmunity may result from the break down of thymic self tolerance mechanisms the mimicry of self proteins by pathogen proteins and from the degeneration of peripheral T cell tolerance Although the exact mechanisms are poorly understood, the generation of autoimmune lesions is thought to require an MHC susceptibility allele capable of binding and presenting the antigens that initiate the autoimmune process; reactive T cells specific for those antigens and, target antigens available for presentation by an MHC susceptibility allele to antigen specific T lymphocytes (Sinha, A.A et al 1990, Science 248 1380) Immune responses initially directed against microbial pathogens are implicated in some autoimmune diseases and autoantibodies specific for a self antigen often also have specificity for viral or bacterial determinants The mechanisms behind this pathogen-associated autoimmune in j ury are poorly understood and the sites of autoimmune damage resulting from pathogenic mfection are often observed at sites distant from the site of mfection The identification of the target antigens of the autoimmune response may permit the rational development of therapies to block the interaction of these antigens with the autoimmune response, for example by the specific deletion of the anti-self T lymphocytes involved Accordingly, there is a need for an assay which can be used to identify the target antigens which are recognised by CTL in the generation of an autoimmune response CTL also play an important defensive role in the mammalian immune response to destroy invading pathogens, such as bacteria, viruses and microorganisms and defective cells, such as cancer cells There is a need for an assay which can be used to identify those epitopes of foreign pathogens, precancerous cells and cancer cells which are recognised by CTL Synthetic peptides have been used in attempts to identify viral and tumor target epitopes recognised by CTL However, misleading results have been obtained as, frequently CTL may recognize only very short segments of the synthetic peptide The acid elution method has been used in attempts to identify the short epitopes presented on the cell surface m association with MHC. The epitopes, once identified, may form the basis for the design of therapeutics for T lymphocyte mediated autoimmune disease, pathogenic infections and tumors In particular, it is expected that the epitopes will be useful for as the basis for vaccines to specifically prime CTL SUMMARY OF THE INVENTION The present inventors have surprisingly found that antigens may be endogenously processed and secreted by the secretory pathway of a cell The secreted antigens were shown to associate with cell surface MHC class I molecules of bystander target cells The present inventors found that virally infected cells or cells transfected with a viral antigen were able to transfer endogenously processed peptides in vitro to recipient target cells for presentation to, and efficient recognition by, primed cytotoxic T lymphocytes (CTL) specific for the virus Importantly, the present inventors demonstrated that the peptide transfer did not involve viral killing or CTL liberation of peptides from infected cells or the release of membrane-bound MHC class I associated antigen from the cell surface. Peptide transfer by the virally infected cells was found to be dependent on the presence of TAP transporters. Surprisingly, peptide transfer was found to be independent of the presence of MHC class I molecules, showing that the transferred peptide is not being transported to the cell surface in association with membrane bound MHC class I molecules. Peptide transfer was shown to occur through active transport via the secretory pathway of the cell and an inhibitor of the secretory pathway was shown to inhibit peptide transfer to target cells. The invention therefore contemplates a method of identifying antigens which are capable of being endogenously processed by a cellular secretory pathway and transferred to bystander target cells for presentation to cytotoxic T lymphocytes, comprising: introducing an antigen into a donor cell lacking in MHC class I molecules; incubating in an in vitro medium the donor cells, primed cytotoxic T lymphocytes having specificity for the antigen, and target cells which express MHC class I molecules and are labelled with a detectable intracellular marker, under suitable conditions such that the donor cells remain intact, and; measuring the amount of detectable marker released into the incubation medium. Preferably the antigen is a viral, bacterial or tumor antigen. In an embodiment the antigen is VSV, an allo peptide or a self peptide. The invention also relates to a method of assaying a medium for the presence of a substance that affects processing of an endogenously processed antigen in a cellular secretory pathway, comprising: incubating in an in vitro medium a donor mammalian cell having an antigen capable of being endogenously processed by a secretory pathway of the donor cell, a target cell expressing MHC class 1 molecules and labelled with a detectable intracellular marker, and primed cytotoxic lymphocytes having specificity for the antigen, in the presence or absence of a substance that is suspected of affecting processing of endogenously processed antigen in the cellular secretory pathway, under suitable conditions such that the donor cells remain intact, and; measuring the amount of detectable marker released into the medium in the presence or absence of the substance. In an embodiment, the donor cell lacks MHC class I molecules. Any cell expressing MHC Class I molecules may be used as the target cell. In a preferred embodiment, mammalian target cells, including human cells, may be used. In a particular embodiment the antigen is VSV and the target cell is RMA. It is expected that substances that affect processing of an endogenously processed antigen in a cellular secretory pathway will be useful as immunomodulators for stimulating or inhibiting the immune response. The invention still further relates to a method of characterizing a tumor or viral antigen comprising: obtaining, from a mammal, tumor cells or virally infected cells and primed cytotoxic T lymphocytes havmg specificity for the tumor or virally infected cells, preparing a cDNA library from the tumor or virally infected cells and expressing the cDNA library in transfected host cells, incubatmg in a medium, transfected host cells, target cells expressing MHC class I molecules and labelled with a detectable intracellular marker and the primed cytotoxic lymphocytes under suitable conditions such that the donor cells remam mtact, and, measuring the amount of the detectable marker released into the medium and identifying those transfected cells capable of causing lysis of the target cells and characterizing the transfected DNA and expression product of the transfected DNA of the identified transfected cells As will be appreciated, the method may also be employed to characterize alloantigens, self antigens and antigens of other pathogens, such as bacteria, protozoa, or other microorganisms or parasites Transiently transfected COS cells may be employed m the methods of the invention The present invention also contemplates the cloning of genes encoding the antigens characterized by the methods of the invention and vaccines incorporating the characterized antigens and epitopes These and other aspects of the present mvention will become evident upon reference to the following detailed description and attached drawings In addition, reference is made herein to various publications, which are hereby incorporated by reference in their entirety DESCRIPTION OF THE DRAWINGS The mvention will be better understood with reference to the drawings in which Figure 1A is a bar graph showing the concentration of VSV-N peptide which yields a particular percentage of cvtotoxicity, Figure IB is a bar graph showing the ability of VSV infected cells (RMA+VSV), cells pulsed with synthetic peptide (RMA+p) and control cells (RMA+-) to generate and transfer peptides to uninfected, chromium laden, target cells, Figure 1C is a bar graph showing the ability of RMA cells infected for 12 hours (RMA + VSV), or treated with VSV immediately prior to adding the effectors (RMA 4 + VSV) to generate and transfer peptides to uninfected, chromium laden, target cells, Figure 2A is a bar graph showing the ability of human Tl, or mouse Ltk-(L), and NIH3T3 (NIH) cells treated with or without (L+-) VSV to generate and transfer peptides to uninfected, chromium laden, target cells, Figure 2B is a bar graph showing that Tl, L, and NIH 3T3, cells were not lysed by VSV specific K b restricted CTL, Figure 2C is a bar graph showing ability of TAP 1 expressmg CMT 1-4 cells to generate and transfer peptides to uninfected, chromium laden target cells, Figure 3A is a bar graph showmg that VSV mfected human T2 and mouse CMT 64 cells are not recognized by VSV specific K b restricted CTL, Figure 3B is a bar graph showmg the effect of heavy and light MHC chains on peptide transfer usmg β2 m deficient R1E cells and R1E cells transfected with K b (RIEKb), and R1E transfected with β2 m and K b (RlEKbb2), with or without VSV treatment, Figure 4 is a bar graph showmg the effect of brefeldin A (BFA) on peptide transfer, Figure 5 is a bar graph showmg that supernatants from Ltk" cells mfected with VSV mediated lysis of RMA cells by VSV specific CTL, Figure 6 is a diagram illustrating the surrogate antigen processing of the mvention and alternative fate of antigenic peptides within the secretory pathway, Figure 7A is a graph showing a CTL control assay from spleen cells not pulsed with VSV-N 52-59 peptide, Figure 7B is a graph showing a CTL assay from spleen cells pulsed with VSV-N 52-59 peptide, Figure 7C is a graph showing a CTL assay from spleen cells pulsed with VSV-N 52-59 peptide, Figure 7D is a graph showing a CTL assay from spleen cells pulsed with VSV-N 52-59 peptide, Figure 8A is a bar graph showing that peptide transfer occurs following infection with a recombinant vaccinia virus encoding the VSV-N 52-59 and is MHC haplotype independent, and Figure 8B is a bar graph showing that peptide transfer occurs in a TAP dependent manner DETAILED DESCRIPTION OF THE INVENTION As hereinbefore mentioned, the present inventors have demonstrated that antigens may be endogenously processed and secreted by the secretory pathway of a cell and cells are able to transfer endogenously processed peptides in vitro to recipient target cells for presentation to, and efficient recognition by, specifically primed CTL The present inventors demonstrated that the peptide transfer is not dependent on viral killmg or CTL liberation of peptides from mfected cells or the release of membrane-bound MHC class I associated antigen from the cell surface Peptide transfer by the virally mfected cells was found to be dependent on the presence of TAP transporters Surprisingly, peptide transfer was found to be independent of the presence of MHC class I molecules, showing that the transferred peptide is not be g transported to the cell surface in association with membrane bound MHC class 1 molecules Peptide transfer was shown to be occurring through active transport via the secretory pathway of the cell and an inhibitor of the secretory pathway was shown to inhibit peptide transfer to target cells. Accordingly as hereinbefore mentioned, the present invention provides a method of identifying antigens which are capable of being endogenously processed by a novel cellular secretory pathway and transferred to bystander target cells for presentation to cytotoxic T lymphocytes, comprising: introducing an antigen into a donor cell lacking in MHC class I molecules; incubating in an in vitro medium the donor cells, primed cytotoxic T lymphocytes having specificity for the antigen, and target cells which express MHC class I molecules and are labelled with a detectable intracellular marker, under suitable conditions such that the donor cells remain intact, and; measuring the amount of detectable marker released into the incubation medium. Antigens which require endogenous processing for presentation to CTL are well known. The majority of cytotoxic T lymphocytes recognize epitopes of MHC class I restricted antigens in association with class I major histocompatibility molecules. Such antigens are usually referred to as MHC class I restricted antigens. MHC class I restricted antigens are those antigens which require endogenous cell processing and whose epitopes are presented on the cell surface by MHC class I molecules for recognition by CTL. Antigens suitable for endogenous processing may be derived from any protein endogenous to a cell, for example from normal, viral, bacterial, defective or tumor proteins. The peptides resulting from endogenous processing of the antigen have the appropriate amino acid sequence for interaction with, and presentation by, MHC class I molecules. The resulting peptides expressed at the cell surface may be detected by CTL and can be extracted from cells, for example by treating whole cells with trifluoroacetic acid. Examples of MHC class I antigens include viral antigens, such as influenza nucleoprotein, vesicular stomatitis virus N protein (VSV-N), tumor antigens, HSV, protozoal, bacterial or parasitic antigens, allopeptides, self peptides, HIV peptides and endogenous peptides and other viral peptides. Examples of MHC class I antigens are provided in Rammensee, et al., 1995 (Immitnogenetics, 41:178-228). A cellular secretory pathway for endogenously processing antigens has not heretofore been described. By cellular secretory pathway is meant a route of intracellular antigen processing, whereby endogenous antigen is degraded in the cell and processed into a peptide which is secreted from the cell in a form suitable for interaction with cell surface MHC class I molecules of a bystander target cell and the translocation system. The cellular secretory pathway may be distinguished from the known routes of endogenous antigen processing, which are dependent on the presence of MHC class I molecules in the cell, as the cellular secretory pathway is not dependent on the presence of MHC class I molecules in the antigen processing cell. The cellular secretory pathway may be further characterized as being dependent on the expression of functional TAP transporters in the cell and it is specifically inhibited by inhibitors of protem secretion, such as Brefeldm A It will be appreciated that compounds which disrupt the Golgi or mterfere with exocytosis of secretory protems will inhibit protein secretion from the cell In particular, the cellular secretory pathway is inhibited by Brefeldm A, preferably at concentrations which inhibit protem secretion but do not harm cell viability, for example from 1 to 10 μg/ml, more preferably from 3 to 6 μg/ml, most preferably about 4 to 5 μg/ml It should be noted that the sensitivity of the cellular secretory pathway to specific inhibitors of protem secretion such as Brefeldm A, clearly distmguishes the cellular secretory pathway from the pathway for processing exogenous antigens, whereby exogenous antigens may become associated with MHC molecules at the cell surface The processmg of exogenous antigens is not affected by Brefeldm A Suitable donor cells for use in the method of the mvention, for identifying antigens which are capable of being endogenously processed by a cellular secretory pathway and transferred to bystander target cells for presentation to CTL, are any animal cells, preferably mammalian cells which are lacking in cell surface expression of MHC class I molecules As noted above, the donor cells lacking m cell surface expression of MHC class I molecules are used m the method of the mvention m order to distinguish the novel cellular secretory pathway from the known pathway of endogenous antigen processing, which depends on the presence of MHC class I molecules, whereby peptide processed from the endogenous antigen is transported to the cell surface only m association with MHC class I molecules However, with suitable controls, it is also anticipated that cells havmg MHC Class I molecules could be used as donor cells Mammalian cells lacking in cell surface expression of MHC class I molecules may be selected by methods known in the art Suitable cells may lack or have a non¬ functional defect m the appropriate genes encodmg MHC class I molecules, have a mutation in the MHC locus, be unable to transport MHC class I molecules to the cell surface or, be deficient in the expression of class I molecules It is also anticipated that non-mammalian cells may be used as donor cells, such as avian, amphibian, reptile, fish and insect cells Donor cells lackmg m cell surface expression of MHC class I molecules mav be selected for example bv infectmg a cell with a recombinant viral vector such as VSV, and testing for lvsis by VSV specific CTL FACS analysis may also be used to detect MHC class I molecules on the surfaces of a putative donor cell The biosynthesis and intracellular transport of MHC class I molecules may also be biochemically characterized For example, endo H which cleaves N-lmked oligosacchaπdes only when they are in the high mannose form characteristic of proteins present in the ER and cis-Golgi complex may be used to measure intracellular transport Pulse-chase methodology mav also be utilized to confirm donor cells lackmg m cell surface expression of MHC class I molecules The above methods are illustrated in part in Restifo, N P et al, (1993, / Exp Med 177 265) Examples of cells which express low levels of MHC class I molecules are tumor cells derived from colon, breast, lung mesothelioma and lung cancers of the small cell histology (See Restifo, N P , supra 1993) Examples of suitable donor cells include cells lackmg in β2-mιcroglobulιn, which is required for the intracellular transport of MHC class I molecules to the cell surface Examples of cells deficient m β2 microglobuhn include murine R1E cells and the human Daudi cell line Tumor cells deficient in cell surface MHC class I molecules may also be used Suitable donor cells mclude human Tl (Salter, R O and P Cresswell, 1986, EMBO J 5 943), murme Ltk-(H-2 K ), NIH 3T3 (H-2 d ), Rl E (Williams et al 1989, / Immunol 142 1796) LCL cells with chromosome deletion, T2 cells and mouse cell lines expressing HLA for allotransfer may be used m the methods of the mvention Preferably, donor cells should be selected which are not defective m other aspects of endogenous antigen processmg, for example the proteasome components and the TAP transporters Cells deficient m certa proteasome or TAP transporter components m addition to MHC class I components may be adapted for use as donor cells by correcting the proteasome or transporter deficiency For example if the cell is deficient in TAP 1 or TAP 2, such as tumor cells, this defect may be corrected by mtroducmg a nucleotide sequence encodmg TAP 1 and /or TAP 2 mto the cell It is also contemplated that the method of the invention for identifying antigens capable of being processed by a cellular secretory pathway and transferred to bystander target cells may be carried out using donor cells which are not necessarily deficient in surface expression of MHC class I as long as suitable controls are performed Suitable controls are those which serve to distinguish the novel cellular secretory from the known MHC class I dependent pathway of endogenous antigen processmg Such controls should be performed to rule out the possibility that MHC class I bound antigens at the surface of the donor cell are released mto the medium to associate with the target bystander cells For example, the cells may be pulsed with synthetic peptide and followed for release or antisense nucleotide molecules may be used to inhibit MHC expression Use of supernatant from VSV infected cells can sensitize cells for VSV specific killmg The antigen to be identified in the method of the invention may be naturalh occurrmg m the donor cell or may be mtroduced mto the donor cell by methods known m the art The antigen may be mtroduced mto the cell by microinjechon The antigen may also be mtroduced mto the donor cell by mtroducmg a nucleic acid molecule encodmg the antigen mto the donor cell under suitable conditions to obtain expression of the antigen m the donor cell The nucleic acid molecule encodmg an antigen may be mtroduced mto the donor cell under the control of a suitable promoter The nucleic acid encoding the antigen may be readily synthesized using techniques known in the art A nucleic acid molecule encoding an antigen may be isolated and sequenced, for example, by synthesizmg cDNAs from RNA and using rapid amplification of cDNA ends (RACE, Frohman, et al , 1988) using oligonucleotides specific for the antigen, and analyzing the sequences of the clones obtamed following amplification Oligonucleotides specific for the antigen may be identified bv comparing the nucleic acid sequence to known sequences of the antigen Nucleic acid molecules encodmg the antigen may also be constructed by chemical synthesis and enzymatic hgation reactions using procedures known in the art The sequence encoding the antigen may also be prepared using recombinant DNA methods Nucleic acid molecules having a sequence which codes for the antigen may be mcorporated m a known manner mto an appropriate expression vector which ensures good expression of the antigen or part thereof Possible expression vectors include but are not limited to cosmids, plasmids, or modified viruses, so long as the vector is compatible with the donor cell used It is contemplated that the nucleic acid molecules described herem contain the necessary elements for the transcription and translation of the mserted protein antigen- sequence Suitable transcription and translation elements may be derived from a variety of sources, including bacterial, fungal, viral, mammalian, or insect genes Selection of appropriate transcription and translation elements is dependent on the donor cell chosen as discussed below, and may be readily accomplished by one of ordinary skill m the art Examples of such elements include a transcπptional promoter and enhancer or RNA polymerase binding sequence, a πbosomal binding sequence, including a translation initiation signal Additionally, depending on the donor cell chosen and the vector employed, other genetic elements, such as an origin of replication, additional DNA restriction sites, enhancers, and sequences conferrmg mducibility of transcription may be mcorporated mto the expression vector It will also be appreciated that the necessary transcπptional and translation elements may be supplied by the nucleic acid molecule encodmg native antigen The nucleic acid molecules may also contain a reporter gene which facilitates the selection of transformed or transfected donor cells Examples of reporter genes are genes encodmg a protem such as β-galactosidase, chloramphenicol acetyltransferase, firefly luciferase, or an immunoglobulin or portion thereof such as the Fc portion of an immunoglobulm preferably IgG In a preferred embodiment, the reporter gene is lac Z Transcription of the reporter gene is monitored by changes m the concentration of the reporter protein such as β-galactosidase, chloramphenicol acetyltransferase, or fireflv luciferase This makes it possible to visualize and assay for expression of the antigen m the donor cell Nucleic acid molecules comprising a sequence encodmg an antigen can be mtroduced into donor cells via transformation, transfection, mfection, electroporation etc Methods for transforming transfecting, etc host cells to express foreign DNA are well known in the art (see, e g , Itakura et al , U S Patent No 4,704,362, Hmnen et al , PNAS USA 75 1929-1933, 1978, Murray et al , U S Patent No 4,801,542, Upshall et al , U S Patent No 4,935,349, Hagen et al , U S Patent No 4,784,950, Axel et al , U S Patent No 4,399,216, Goeddel et al , U S Patent No 4,766,075, and Sambrook et al Molecular Cloning A Laboratory Manual, 2nd edition, Cold Sprmg Harbor Laboratory Press, 1989, all of which are mcorporated here by reference) Suitable expression vectors for directing expression in mammalian donor cells generally mclude a promoter, as well as other transcπptional and translational control sequences Common promoters mclude SV40, MMTV, metallothιoneιn-1, adenovirus Ela, CMV, immediate early, immunoglobulin heavy cham promoter and enhancer, and RSV- LTR Protocols for the transfection of mammalian cells are well known to those of ordmary skill in the art Representative methods include calcium phosphate mediated electroporation, retroviral, and protoplast fusion-mediated transfection (see Sambrook et al , supra) In an embodiment, the nucleic acid molecule may be mtroduced mto the donor cell in a viral vector, preferably a vaccinia viral vector, most preferably attenuated Suitable promoters for use with vaccmia viruses mclude P7 5 (Cochran, M A et al, 1985, / Virol 54 30), Pll (Bertholet, C et al, 1985, Proc Natl Acad Sc USA, 82 2096), CAE-1 (Patel, D D et al, 1988, Proc Natl Acad So USA 85 9431) A vaccmia virus contaming the nucleic acid molecule may be prepared by homologous recombination In an embodiment, a nucleic acid molecule encodmg a VSV peptide may be hgated mto a vaccmia virus shuttle vector such as pJS5 The vector may be transfected into cells that have been infected with vaccmia virus so that the nucleic acid encodmg the VSV peptide is inserted mto the vaccmia virus by homologous recombination The nucleic acid molecule may be mserted mto a non-essential site of a vaccmia viral vector Such non-essential sites are well known and are described, for example, m Perkus et al, 1986, Virology 152 285, Hruby et al, 1983, Proc Natl Acad Set USA 80 3411 and, Weir and Moss, 1983, / Virol 46 530 Recombinant viruses expressmg the antigen may be readily identified usmg techniques known m the art and discussed, for example, m Moss, B, 1992, (Curr Topics Microbiol Immunol 158 25) It will be appreciated that the antigen mav also be an endogenous antigen which naturally occurs in the donor cell, for example an endogenous tumor antigen Antigens of intracellular pathogenic agents may be mtroduced into the donor cell by infecting the donor cell with the pathogenic agent Examples of mtracellular pathogenic agents mclude viruses, such as HIV, influenza virus, herpes virus, papilloma virus, retroviruses, and πckettsia, and intracellular bacterial pathogens, and protozoan parasites, such as malaria, trypanosomes and Leishmania Pπmed CTL havmg specificity for the antigen may be obtamed by known methods Specific primed cytotoxic T lymphocytes may be obtamed from the spleen, blood or lymph of a mammal infected with an mtracellular pathogen or hav g a tumor expressmg the antigen of mterest Primed CTL having specificity for the antigen may also be obtamed by immunizing a mammal with the antigen Methods for immunization of mammals to prime cytotoxic immunity are described for example m Rock, K L et al (1993, / Immunol 150 1244) and Moss, B (1990, Seminars in Immunol 2.317) Suitable immunization protocols mclude administering the antigen or peptide fractions thereof together with β2-mιcroglobulιn, or admmistenng a recombmant viral vector encodmg the antigen or peptide fractions of the antigen Suitable target cells for use in the method of the invention include mammalian cells which express MHC class 1 molecules on their surface The target cells and primed CTL may be selected from the same mammal It will be appreciated that target cells should be selected which do not themselves express the antigen of mterest For example where the antigen of mterest is a tumor antigen or an intracellular pathogenic agent of a mammal, a suitable target cell may be selected from non-tumor cells or uninfected cells of the mammal It will be appreciated that cells, other than those from the mammal may also be used, for example, cells from another animal may be transfected with the appropriate MHC Suitable mcubation media and conditions for maintaining mammalian cells mtact during in vitro culture are well known The target cells are labelled with a detectable mtracellular marker The mtracellular marker should be one which is not readily released from the target cell in the absence of cell damage and lysis of the target cell Suitable mtracellular markers mclude I 51 lCr, fluorescent markers or Beckman reagent An antigen which is endogenously processed by a cellular secretory pathway and transferred to bystander target cells for presentation to cytotoxic T lymphocytes may be identified based on the release of detectable mtracellular label mto the medium, or based on some other measure of the death of target cells, for example based on the detection of thymidase or DNA fragments As hereinbefore noted, the present invention also provides a method of assaymg a medium for the presence of a substance that affects processmg of an endogenously processed antigen m a cellular secretory pathway, comprising incubating m an in vitro medium a donor mammalian cell havmg an antigen capable of bemg endogenously processed by a secretory pathway of the donor cell, a target cell expressmg MHC class I molecules and labelled with a detectable intracellular marker, and primed cytotoxic lymphocytes havmg specificity for the antigen, in the presence or absence of a substance that is suspected of affecting processing of endogenously processed antigen in the cellular secretory pathway, under suitable conditions such that the donor cells rema mtact, and, measurmg the amount of detectable marker released into the medium m the presence or absence of the substance It will be appreciated that substances which stimulate or inhibit the processmg of an endogenously processed antigen in a cellular secretory pathway will, m the method of the mvention, result m mcreased or decreased amounts respectively of detectable marker being released mto the medium over a certain time period compared to control incubations carried out m the absence of the substance The method may be used to specifically assay for substances that affect processmg, rather than secretion, of the endogenously processed antigen in the cellular secretory pathway by simultaneously determmmg the effect of the substance on secretion of a protein that is known to be secreted from the donor cell, such as growth hormone for example The method of the mvention may therefore be used to identify substances which inhibit or stimulate processing of antigen from a cell by the cellular secretorv pathway It is anticipated that substances which inhibit or stimulate antigen secretion by the cellular secretory pathway will have an lmmunomodulating effect when administered to a mammal in vivo and be useful as pharmaceuticals for treating rumors and mfection with mtracellular pathogens such as viruses Substances which inhibit antigen processing may also be useful for the treatment of autoimmune diseases, particularly diseases of MHC class I associated autoimmune disorders such as the HLA-B27 related spondyloarthropathies, including ankylosing spondyhtis, Reiter's syndrome and reactive arthropathy, Sjogren s syndrome, rheumatoid arthritis, type I polyendocrme failure, multiple sclerosis, hypothyroidism, Hashimoto s disease, Graves disease and psoriasis vulgaπs, and autoimmune resultmg from an excessive immune response to foreign antigens of mtracellular pathogens Substances which inhibit antigen secretion may also be useful for the treatment of viral mfections where the antigenic load is high, in order to prevent exhaustion and depletion of CTL, leading to the development of immunological tolerance Substances which inhibit antigen secretion may also be used to treat organs prior to transplant to avoid rejection and to treat mammals with transplanted tissue or organs to decrease the immune rejection response Substances which stimulate antigen secretion may be useful for the treatment of tumors and for mtracellular pathogens, where the tumor or pathogenic antigens are generally sequestered from the immune system and not appropriately presented on the cell surface bv MHC class I molecules As hereinbefore mentioned, the mvention still further provides a method ot characterizing a tumor or viral antigen capable of bemg endogenously processed by a cellular secretory pathway compπsmg obtammg, from a mammal, tumor or virally infected cells and primed cytotoxic T lymphocytes havmg specificity for the tumor or virally infected cells; preparing a cDNA library from the tumor or virally infected cells and expressing the cDNA library in transfected host cells; incubating in a medium, transfected host cells, target cells expressing MHC class I molecules and labelled with a detectable intracellular marker and the primed cytotoxic lymphocytes under suitable conditions such that the donor cells remain intact, and; measuring the amount of the detectable marker released into the medium and identifying those transfected cells capable of causing lysis of the target cells and characterizing the transfected DNA of the identified transfected cells. The mammal from which the rumor cells and primed CTL are obtained may be a mammal having a naturally occurring tumor or a tumor induced by means of a tumorigenic substance, or may be a mammal into which a tumor has been mtroduced. The mammal may be a human having a tumor and may also be a non-human mammal into which a human tumor or human rumor cell line has been introduced. Human tumor cell lines are well known, see for example Restifo, N. B. et al. (1993) /. Exp. Med. 177:265. cDNA expression libraries may be constructed, following known techniques, using mRNA from tumor cells. cDNA clones coding for antigens processed by the cellular secretory pathway may be identified, subcloned and sequenced. The cDNA sequences, or fragments thereof may be used as hybridization probes to isolate genomic DNA encoding for the antigen. The invention therefore provides a method for cloning the gene encoding the tumor antigen and for isolating the antigen. Identification and isolation of the antigen will permit studies of the role of the antigen in the development of the cytotoxic T lymphocyte immune response to the tumor and the development of substances which affect the development of this immunity as stimulators or inhibitors of the immune response. Vaccines may be developed using the genes, for example in retroviral vaccines. The antigens expressed may also be used as the basis for vaccines. It is contemplated that the antigens will bind to the empty MHC 2 molecules which occur on many tumors to aid immune recognition of the tumors. Nucleic acid molecules encoding tumor antigens may be isolated and sequenced and incorporated into a recombinant molecule which ensures good expression of the antigen or part thereof. In general, a recomb nant molecule of the invention contains a nucleic acid molecule, or an oligonucleotide fragment thereof, encoding the antigen and an expression control sequence operatively linked to the nucleic acid molecule or oligonucleotide fragment. A nucleic acid molecule of the invention or an oligonucleotide fragment thereof, may be incorporated into a plasmid vector, for example, pECE. Suitable regulatory elements may be derived from a variety of sources, including bacterial, fungal, viral, mammalian, or insect genes. Selection of appropriate regulatory elements is dependent on the host cell chosen, and may be readily accomplished by one of ordinary skill in the art. Examples of regulatory elements include: a transcriptional promoter and enhancer or RNA polymerase binding sequence, a ribosomal binding sequence, including a translation initiation signal. Additionally, depending on the host cell chosen and the vector employed, other genetic elements, such as an origin of replication, additional DNA restriction sites, enhancers, sequences conferring mducibility of transcription, and selectable markers, may be incorporated mto the expression vector The antigen or parts thereof, may be obtamed by expression in a suitable host cell using techniques known in the art Suitable host cells include prokaryotic or eukaryotic organisms or cell lmes, for example, yeast, E coli and mouse NIH 3T3 cells may be used as host cells The protem or parts thereof may be prepared by chemical synthesis using techniques well known m the chemistry of protems such as solid phase synthesis (Merπfield, 1964, J Am Chem Assoc 85 2149-2154) or synthesis m homogenous solution (Houbenweyl, 1987, Methods of Organic Chemistry, ed E Wansch, Vol 15 I and II, Thieme, Stuttgart) DNA sequences encodmg the antigen, or a part thereof, may be expressed by a wide variety of prokaryotic and eukaryotic host cells, including bacterial, mammalian, yeast or other fungi, viral, plant, or sect cells Methods for transformmg or transfecting such cells to express foreign DNA are well known m the art (see, e g , Itakura et al , U S Patent No 4,704,362, Hinnen et al , PNAS USA 75 1929-1933, 1978, Murray et al , U S Patent No 4,801,542, Upshall et al , U S Patent No 4,935,349, Hagen et al , U S Patent No 4,784,950, Axel et al , U S Patent No 4,399,216, Goeddel et al , U S Patent No 4,766,075, and Sambrook et al Molecular Cloning A Laboratory Manual, 2nd edition, Cold Spring Harbor Laboratory Press, 1989, all of which are mcorporated herein by reference) Preferably, the tumor antigen is expressed in a mammalian cell Mammalian cells suitable for carrying out the present mvention include, among others COS (e g , ATCC No CRL 1650 or 1651), BHK (e g , ATCC No CRL 6281), CHO (ATCC No CCI 61), HeLa (e g , ATCC No CCL 2), 293 (ATCC No 1573) L cells, NIH 3T3 cells and NS-1 cells Suitable expression vectors for directmg expression in mammalian cells generally mclude a promoter, as well as other transcπptional and translational control sequences Common promoters include SV40, MMTV, metallothιoneιn-1, adenovirus Ela, CMV, immediate early, immunoglobulin heavy chain promoter and enhancer, and RSV-LTR Protocols for the transfection of mammalian cells are well known to those of ordmary skill in the art Representative methods include calcium phosphate mediated transfection, lipofection-mediated transfection, electroporation, retroviral, and protoplast fusion- mediated transfection (see Sambrook et al , supra) The tumor antigen may be prepared by culturmg the host/vector svstems descπbed above, m order to express the recombmant antigen It will be appreciated that the method of the invention may also be used to characterize antigens, other than tumor antigens, that are capable of bemg endogenously processed by a cellular secretory pathway For example, the method will also be useful for characterizing antigens of mtracellular pathogens, such as viruses, bacteria and protozoa The expression product of the transfected DNA of those transfected host cells capable of causmg lysis of the target cells may be directly characterized Lysis results from CTL recognition of an antigenic peptide presented by the MHC class I molecules on the surface of the target cells Accordmgly, the MHC class I molecules with their associated antigenic peptides may be isolated from the incubation medium and the antigenic peptide extracted using known techniques, as described, for example in Van Bleek, G M and S G Nathenson, 1990, Nature 348 213, and Rammensee et al , 1995, lmmunogenettcs, 41 178-228 The antigenic peptides identified may be the dommant T cell epitope of the antigen The MHC Class I fractions may be isolated from the mcubation medium by immunoprecipitation and a low molecular weight peptide fraction may be extracted by acid denaturation followed by centπfugation The low molecular weight peptide fraction may be separated by reverse phase HPLC The prominent peak corresponding to the antigenic peptide recognised by the T cell receptor may be identified by comparison with the profile obtamed from control target cells mcubated with non-transfected donor cells The promment peak corresponding to the antigenic peptide may be further analyzed, for example by microsequencing Antigenic peptides may also be identified by peptide scan methods or deletion analysis The DNA of those transfected cells capable of causing lysis of the target cells may also be characterized In an embodiment, all of the transfected host cells capable of causmg lysis may be pooled, their DNA extracted and retransfected mto host cells for one or more further rounds of screening Ultimately a single transfected DNA sequence, expressing antigen capable of causmg lysis, may be identified, sequenced and expressed for further characterization by sequencing The method of the invention thus provides a method for identifying the antigenic peptide or epitope from tumor or viral antigens recognised by the T cell receptor All publications, patents and patent applications referred to herein are incorporated by reference m their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety T e following examples are offered by way of illustration only, and not by way of limitation EXAMPLES Example 1 Virally infected RMA cells transfer peptides to recipient chromium labelled uninfected RMA cells. A system for investigating surrogate antigen processmg was developed usmg the immunodominant epitope of the VSV-N protem (VSV-N 52-59) presented by the K b molecules of the MHC class I CTL effectors specific for VSV were generated by injecting C57BL/6 mice with VSV (5xl0 6 TCID 50 per site) m the ears and/or foot pads Five days later the drammg lymph nodes were removed and lymphocytes cultured at 4xl0 6 cells/ml for 3 days m the absence of stimulation Cold targets were prepared as follows RMA cells were treated with VSV (MOIof 2) for 12 to 18 hours, and subsequently washed three times Hot targets were prepared as follows RMA cells were treated with [ 51 ]Cr for two hours and subsequently washed three times Cold targets were diluted two-fold starting with 2xl0 5 cells per well Each well received lxlO 6 effectors and lxlO 4 hot targets Effectors, cold and hot targets were mcubated together in V-bottom 96-well plates for 4 hours, followed by removal of lOOμl of supernatant which was then counted for radioactivity released Figure 1A shows the concentration of VSV-N peptide which yields a particular percentage of cytotoxicity Fifty percent of maximum chromium release was yielded at a peptide concentration of 50 pM This result was highly reproducible In order to determme the RMA dose response, the effector cells were tested agamst hot RMA target cells pulsed with peptide VSV N52-59 at different concentrations for 2 hours m a dose response The effector cell population was tested agamst hot RMA target cells pulsed with peptide VSV N52-59 at different concentrations for 2 hours in a dose response The ability of cells to process and transfer the generated peptides to uninfected cells was addressed by mfectmg cells not labelled with chromium and then incubating these cells together with unmfected target cells which were chromium laden (Figure IB) When wild- type RMA infected cells were used as the peptide donors we found that unmfected RMA cells were killed efficiently, greater than 25% at a cold/hot of 20/ 1 It is suggested from our standard curve that the chromium labelled target cells had been exposed to a mmimum of 10-50 pM of peptides derived from the VSV mfected cells In figure IB, a control in which the surrogate cells were pulsed with a high concentration of synthetic peptide was included to examine if the peptide could be exchanged from the plasma membrane to trigger recognition by T cells on bystander targets Cells pulsed with saturatmg concentrations of antigenic peptides were found to not be able to efficiently transfer peptides to recipient cells Although this may happen over time, as suggested by Huang et al (A Y C Huang, et al Science 264 961, 1994), we found no evidence for this m our assays. RMA cells were mfected for 12 hours (RMA + VSV) prior to the CTL assay, or alternatively, RMA cells were treated with VSV immediately prior to adding the effectors (RMA 4 + VSV) The effector to target ratios ranged from 12 5 1 to 100 1 The time frame of the assay (4h) did not allow chromium loaded cells to be mfected with VSV, process proteins, and subsequently be recognized by specific CTL (Figure 1C) In addition, our results usmg cold target competition assays indicate that the peptide transferred was the immunodominant VSV-N 52-59 peptide (data not shown) In summary, virally mfected RMA cells were shown to be extremely efficient at transferrmg peptides to recipient chromium labelled unmfected RMA cells Example 2 Ability of human Tl (HLA-A,B,C), murine Ltk- (H-2 k ) and NIH 3T3 (H-2 d ) cells to transfer peptides to recipient RMA cells In order to examme if the underlymg mechanism for transfer mvolves viral killing or CTL liberation of peptides from infected cells, we studied cells which are not recognized by VSV specific K b restricted CTL but retain their ability to be mfected by virus We exam ed the ability of human Tl (R D Salter and P Cresswell EMBO ] 5 943, 1986), murine Ltk " (H-2 k ), and NIH 3T3 cells (H-2 d ) to transfer peptides to recipient unmfected RMA (H-2 b ) cells CTL were generated as described m Example 1 herein, Tl, Ltk-(L), NIH3T3 (NIH) and CMT.1-4 cells were treated with or without VSV (MOI of 2) for 12 to 18 hours and subsequently washed 3 times Hot targets were RMA cells treated with (51 JCr for 2 hours and subsequently washed 3 times The results are shown m Figures 2A and 2C In order to determme the CTL response T1,L, and NIH + VSV, Tl, Ltk-(L), and NIH3T3 (NIH) cells were added to effectors after VSV mfection and ' 51 lCr labelling to examme whether or not they are recognized as targets for VSV-specific CTL at different effector to target ratios (E T) Mock mfected Ltk- cells (L+— ) were used as negative controls In figure 2B, we show that Tl, NIH 3T3 and L cells are not lysed by VSV specific K b restricted CTL However, each of these cell types can transfer peptides to RMA cells (Figure 2A) Accordingly, the wild-type Tl cells, though not recognised by VSV specific CTL are able to transfer a VSV peptide Example 3 Role of TAP transporters and MHC class I molecules in peptide transfer In order to establish the significance of individual components of the antigen processmg pathway for peptide transfer, we mvestigated whether cell variants deficient in one or more elements of this pathway are able to transfer peptide Human T2 (R D Salter and P Cresswell EMBO } 5 943, 1986) and murme CMT 64 (H-2 b ) cells (Jeffenes W A , et al / Immunol 151 2974, 1993 and Gabathuier et al , 1994, J Exp Med 180 1415) do not express either TAP-1 or TAP-2 The effect of TAP on peptide transfer was mvestigated as follows CTL were generated as described in Example 1 Hot targets were RMA cells treated with ' 51 lCr for 2 hours and subsequently washed 3 times Cold targets servmg as peptide donors consisted of RMA, CMT 64+/-IFN-γ (CMT, and CMT IFN respectively), and CMT 64 cells transfected with the rat TAP 1 and TAP 2 genes (CMT1 2) These cells were treated with VSV (MOI of 2) for 12 to 18 hours and subsequently washed 3 times Effectors, cold and hot targets were then mcubated together m V-bottom 96-well plates for 4 hours, followed by removal of 100 μl of supernatant which was then counted for radioactivity released Following mfection with VSV, we found that neither the T2 or CMT 64 cell lmes were recognized by VSV specific K b restricted CTL (Jeffeπes W A , et al / Immunol 151 2974, 1993) (data not shown) and were unable to efficiently transfer peptides to the recipient RMA cells as shown m Figure 3A As noted above, the wild-type Tl cells, though not recognized by VSV specific CTL (Figure 2B), are able to transfer a VSV peptide (Figure 2A) CMT 64 cells induced with IFN-γ to express TAP-1 and -2 were able to transfer peptιde(s) (Figure 3A) Furthermore, CMT 64 transfected with the rat TAP-1 and TAP-2 genes were able to transfer a peptide to RMA cells (Figure 3A) These data demonstrate that mfected cells which lack TAP transporters cannot transfer peptιde(s) These results reveal a dependency on the expression of the TAP transporters for the exchange of peptιde(s) and suggest that the peptides must be translocated mto the ER m order to be transferred In order to investigate whether interaction with/and transport by MHC class I molecules is a prerequisite for peptide transfer, we exammed whether murme Rl E (H-2 k ) cells (D B Williams, et al / Immunol 142 2796, 1989) which lack β-2 microglobulin and therefore do not transport MHC class I heavy cha are able to transfer a VSV peptide The effect of heavy and light chams on peptide transfer was mvestigated as follows CTL were generated as described in Example 1 Hot targets were RMA cells treated with I 51 lCr for 2 hours and subsequently washed 3 times Cold targets consisted of RIE cells and their derivatives RIE transfected with K b (RIEKb), and RIE transfected with β2 m and K b (RlEKbb2) Cold targets were treated with or without (RIEKbb2+-) VSV (MOI of 2) for 12 to 18 hours and subsequently washed 3 times Effectors, cold and hot targets were then mcubated together m V-bottom 96-well plates for 4 hours, followed by removal of 100 μl of supernatant which was then counted for radioactivity released The results are shown m Figure 3B Rl E cells were able to efficiently transfer peptides to RMA cells as shown in Figure 3B The contribution of K b molecules alone and of K b and β-2 microglobulin to this peptide transfer was established by examining Rl E cells derivatives transfected with heavy or heavy and light chains The expression of K b alone and both K and beta-2 microglobulin did not affect the efficiency of peptide transfer of transfected Rl E cells (Figure 3B) This indicated that peptide binding and transport of MHC class I molecules was not required for transfer of the peptide E*arrφk 4 Effect of Brefeldin A on Peptide secretion from infected cells In order to address whether peptides are secreted from mfected cells we used brefeldm A, an inhibitor of protein secretion Brefeldm A was used at concentrations reported previously (Jeffenes W A , et al / Immunol 151 2974, 1993, Nuchtern, J G , et al Nature 339 223, 1989, Yewdell, J W and J R Bennink Nature 339 223, 1989) which effectively inhibit protein secretion but does not harm cell viability Brefeldm A treated RMA cells were infected with VSV and examined for their ability to transfer peptide to recipient cells The effect of BFA on peptide transfer was mvestigated as follows CTL were generated as m Example 1 Hot targets were RMA cells treated with I 51) Cr for 2 hours and subsequently washed 3 times Cold RMA cells were treated with RMA BFA) or without brefeldm A lOμg/ml (BFA) and/or VSV (MOI of 2) for 12 to 18 hours and subsequently washed 3 times Effectors, cold and hot targets were then incubated together in V-bottom 96 well plates for 4 hours, followed by removal of 100 μl of supernatant which was then counted for radioactivity released We found that brefeldin A dramatically inhibited peptide transfer as shown in Figure 4, suggestmg that the peptide must be actively transported through the secretory pathway for efficient transfer to take place This was also confirmed by our observation that growth media from Ltk cells infected with VSV and collected before virally mduced lysis could sensitize unmfected RMA cells for CTL recognition Example 5 Supernatants from VSV infected Ltk' cells sensitize recipient uninfected RMA cells for VSV specific CTL recognition Cold targets were replaced with supernatant of VSV infected cells m the assay described in Example 1 Supernatant of VSV mfected cells was collected 12 hours post mfection, before any virally induced lysis, desalted through a G 25 column and lyophilized before reconstitution m 2 ml of medium to achieve a 5 x concentration The supernatant was added to ' 51) Cr laden RMA cells and the resultmg CTL mediated lysis measured The l 51 iCr release was measured by an LKB1282CS Compugamma counter and the specific l 51 lCr release calculated as (experimental-media control) /(total-media control) x 100% The spontaneous release never exceeded 20% of the maximum release (treated with 1 5% Triton X-100) The results are shown in Figure 5, which shows that the supernatant from the VSV mfected cells sensitized unmfected RMA cells for CTL recognition These results mdicate that contact between the donor and recipient cells is not necessary for sensitization of target cells for CTL lysis Example 6 Ltk cells transfected with VSV-N transfer peptides to recipient non-transfected RMA cells In order to confirm that the transfer of endogenously processed VSV peptides to recipient target cells for recognition by primed CTL was not an anomaly resulting from the effects of VSV infection of the donor cells, donor cells transfected with VSV-N peptides were prepared. Murme Ltk cells (L cells) were transfected with VSV-N to provide a more controlled and convenient system for the screening of substances that affect the endogenous processing and secretion of antigens. L cells were transfected as follows. Approximately 1.0 μg of DNA encoding VSV-N and the neophosphotransferase gene was transfected into L cells with lipofectin (2 μl). The transfected cells were selected in geneticin (G418) lmg/ml for 4 days and to half the concentration of G418 thereafter. Isolated clones were selected, grown and frozen. The transfected clones were collected after the cells reached confluency. The supernatants were then checked for their ability to sensitize uninfected target cells for recognition and lysis by VSV specific CTL. The following results, expressed in per cent lysis of RMA target cells, were obtained from 7 different clones: L VSV-N 1 44%, L VSV-N 2 49%, L VSV-N 3 38%, L VSV-N 4 35%, L VSV-N 8 37%, L VSV-N 11 35%, and L VSV-N 13 37%. The transfection of VSV-N was found to be sufficient to sensitize target cells for CTL killing. Transfected L cells were not recognised directly by the VSV specific cells for CTL killing in the C57B1/6 mouse (H-2b), but were able to transfer the VSV-N peptide to target cells expressing the K b molecule. This confirms that endogenous proteins can produce peptides which are secreted and transferred to target cells. Example 7 Reactivity of CTL after first stimulation The CTL assay described in example 6 was repeated using spleen cells Spleen cells from mice (C57 Bl/6) infected with VSV were kept m culture for 10 days before restimulation in the presence (spl+) or absence of peptides (spl-). The peptide was VSV-N 52-59 at 1 μM. Restimulation of CTL was done with splenocytes from uninfected C57- Bl/6 mice pulsed with 1 μM of the same peptide (spl-+, spl ++) or without peptide (spl--, spl +-) and irradiated for 30 min. prior to addition for restimulation. Targets for the CTL assay are RMA cells pulsed with 1 μM of VSV-N 52-59 peptides and loaded with ! 51 lCr. Figure 7A, shows that splenocytes must be primed with the peptide and restimulated with peptide pulsed cells in order to recognize efficiently VSV-N 52-59 specific targets. Figures 7B, 7C and 7D show that peptides are necessary and must be present for restimulation and in the previous incubation for the generation of specific CTL. Ex ^ ample 8 Peptide transer is not due to VSV replication pathway In order to demonstrate that the life cycle of VSV was not specifically leading to the release of the VSV-N 52-59 peptide, a recombmant vaccmia virus encodmg the VSV-N-52-59 peptide was prepared Vaccmia Virus Constructs The recombmant Vaccmia Virus contammg the VSV-N 52-59 peptide gene (VV-N) was constructed by first hgating a double stranded synthetic oligonucleotide encodmg the VSV-N 52-59 peptide mto a Vaccmia Virus shuttle vector pJS5 (provided by Dr B Moss, NIH, Bethesda, MD) The oligonucleotide, which contamed Sacl and Nhel overhangs for gation mto pJS5, is as follows 3'-tcgag-tac-tct-cct-ata-cag-atg-gtt-ccg-gag- act-cgatc-5' with the complementary strand as 5 -c-atg-aga-gga-tat-gtc-tac-caa-ggc-ctc- tga-g-3' The VV-N peptide was then made by homologous recombination when the pJS5- VSVN52-59 construct was transfected mto CV-1 cells infected with Vaccmia Virus (WR strain provided by Dr S Gillam, Vancouver,B C ) VV-N peptide was plaque purified three times in BS-C-1 cells, then amplified m Hela S3 cells before virus purification on a sucrose gradient (Earl and Moss, 1992) The shuttle vector pJS5 without an msert was also cloned mto Vaccmia Virus (W-pJS5) to provide a vector only control Vaccmia Virus Infection Vaccinia virus mfection of cold targets was done as follows VV-N peptide and W-pJS5 were trypsmized m 0 1 volume 25 mg/ml trypsin (Difco 1 250) for 30 mmutes at 37°C, vortexmg every 10 minutes After washing CMT 64, CMT 12-12, and L cells with phosphate buffered saline (PBS) the virus was added to the cells (MOI of 10) along with 0 5 ml RPMI supplemented with 2% heat mactivated FBS, glutamme, penicillin, streptomycin and Hepes They were mcubated 1 5 hours at 37°C before addmg 3 ml RPMI supplemented with 10% heat inactivated FBS, glutamme, penicillin, streptomycm and Hepes The cells were removed with versene and 0 5% trypsin, washed 2 times with media before addition to the CTL assay The total mfection time was 6 hours until the start of the assay or longer as indicated Hot targets, which were RMA cells or as mdicated, were treated with l 51 'Cr for 1 hour at 37°C and subsequently washed l 51 lCr was used at lOOmCi per 1x106 cells Cold targets were diluted two-fold or three-fold when mdicated startmg with 2x10 5 cells per well at 20 1 ratio cold versus hot targets Each well received lxlO 6 effector CTL and 1x104 hot targets (100 1 ratio effector versus hot targets) Effectors plus cold and hot targets were mcubated together in V-bottom 96-well plates for 4 hours for radioactivity release In some assays, where mdicated, cold targets are replaced by medium contammg peptides or viruses The supernatant of VSV mfected cells was previously desalted through a G 25 column and lyophilized before reconshtution m 2 ml of medium to achieve a 5 x concentration The I 51 lCr release was measured by an LKB1282CS Compugamma counter and the specific t 51) Cr release calculated as (experimental-media control)/ (total-media control) x 100% The spontaneous release never exceeded 20% of the maximum release (treated with 1 5% Triton X-100) In the initial experiment, Ltk" (H-2 k ) were mfected and exammed for the ability of VSV, vaccmia virus (VV) and vaccmia virus encoding VSV-NP 52-59 to transfer the VSV-N peptide to recipient cells The results, shown in Figure 8A, demonstrate that only VSV itself and the VV-N peptide construct are able to transfer the peptide to RMA cells The activities of both viruses are titratable and the VSV appears to be more efficient than the lecombinant VV at transferrmg the peptide These results demonstrate that in a heterologous system, VSV-N peptide expression results in peptide transfer These results also demonstrates that VSV-N peptide expression is not dependent on the expression of the H-2b haplotype Furthermore, this data also demonstrates that an 8 amino acid peptide can transfer the activity Thus it is not dependent on extracellular cleavage events In order to investigate the TAP dependency of peptide transfer in the heterologous expression system, CMT 64 or CMT 64 cells expressmg TAP-1 /2 (CMT 12-12) were infected with the VV-N peptide or with the wild-type VV The results, shown m Figure 8B, demonstrate the TAP dependency of the transfer of VSV-N peptide expressed m a heterologous system These data confirm that the peculiarities of the VSV life cycle do not contribute to the peptide transfer mechanism The results usmg a vaccmia virus construct expressing the VSV-N peptide epitope demonstrate clearly that the peptide transfer is not the result of the VSV replication pathway The vaccmia virus encoded peptide (VSV-N 52-59) also requires a functional TAP transporter, strongly supporting the view that the normal antigen peptide processmg machmery is required In addition, these data demonstrate that the 8 aa peptide itself can be transferred and transfer is thus not dependent on the proteolytic cleavage of a larger fragment extra or mtracellularly Overall, peptide transfer is not dependent on VSV mfection, or the viral lytic cycle

Claims: WE CLAIM: 1 A method of identifying antigens which are capable of being endogenously processed by a cellular secretory pathway and transferred to bystander target cells for presentation to cytotoxic T lymphocytes, comprising mtroducmg an antigen into a donor cell lackmg m MHC class I molecules, mcubatmg in an in vitro medium the donor cells, primed cytotoxic T lymphocytes having specificity for the antigen, and target cells which express MHC class I molecules and are labelled with a detectable intracellular marker, under suitable conditions such that the donor cells remam mtact, and, measuring the amount of detectable marker released mto the mcubation medium 2 A method as claimed in claim 1 wherein the antigen is a viral, bacterial, protozoan, or tumor antigen 3 A method as claimed m claim 1 wherem the antigen is a self antigen or an alloantigen 4 A method as claimed in claim 2 wherem the antigen is VSV 5 A method as claimed in claim 1 wherem the donor cell is transfected with a nucleic acid molecule encodmg the antigen 6 A method as claimed m claim 1 wherem the donor cell is Rl E(H-2 k ) or a Daudi cell 7 A method as claimed in claim 1 wherein the detectable marker is I 51 'chromium 8 A method of assaying a medium for the presence of a substance that affects processmg of an endogenously processed antigen m a cellular secretory pathway, compπsmg mcubatmg in an in vitro medium a donor mammalian cell having an antigen capable of bemg endogenously processed by a secretory pathway of the donor cell, a target cell expressmg MHC class I molecules and labelled with a detectable intracellular marker, and primed cytotoxic lymphocytes havmg specificity for the antigen, in the presence or absence of a substance that is suspected of affecting processmg of endogenously processed antigen in the cellular secretory pathway, under suitable conditions such that the donor cells remam intact, and, measurmg the amount of detectable marker released into the medium m the presence or absence of the substance 9. A method as claimed m claim 8 wherem the donor cell lacks MHC class I molecules 10 A method as claimed in claim 9 wherem the antigen is VSV-N protem and the donor cell is RMA. 11 A method of characterizing a tumor or viral antigen capable of being endogenously processed by a cellular secretory pathway comprising obtaining, from a mammal, tumor cells or virally infected cells and primed cytotoxic T lymphocytes having specificity for the tumor or virally mfected cells; preparmg a cDNA library from the tumor or virally infected cells and expressing the cDNA library in transfected host cells, mcubatmg m a medium, transfected host cells, target cells expressmg MHC class I molecules and labelled with a detectable mtracellular marker and the primed cytotoxic lymphocytes under suitable conditions such that the donor cells remam tact, and; measurmg the amount of the detectable marker released mto the medium and identifying those transfected cells capable of causing lysis of the target cells and characterizing the transfected DNA or expression product of the transfected DNA of the identified transfected cells.

Inventors: ALIMONTI JUDIE, GABATHULER REINHARD, JEFFERIES WILFRED A, KOLAITIS GERASSIMOS, REID GREGOR S D, ALIMONTI, JUDIE, GABATHULER, REINHARD, JEFFERIES, WILFRED, A., KOLAITIS, GERASSIMOS, REID, GREGOR, S., D.
Applicants: ALIMONTI JUDIE, GABATHULER REINHARD, JEFFERIES WILFRED A, KOLAITIS GERASSIMOS, REID GREGOR S D, UNIV BRITISH COLUMBIA, ALIMONTI, JUDIE, GABATHULER, REINHARD, JEFFERIES, WILFRED, A., KOLAITIS, GERASSIMOS, REID, GREGOR, S., D., UNIVERSITY OF BRITISH COLUMBIA
Application year: 1997
Main classification: G01N33
IPC codes: C12N5, C12N15, G01N33, G02B6