Isolation and characterization of antigen-Ia complexes involved in T cell recognition

Lack of binding of peptides carrying the human platelet antigen 1 (HPA-1) dimorphism to purified HLA-DRw52a molecules

The strong association between anti-HPA-1a alloimmunization and DR3, DRw52a phenotype in HPA-1b homozygous women suggests that these class II molecules play a crucial role in the immune response against HPA-1a. The diallelic system HPA-1 results in a single amino acid polymorphism at the residue 33 of the glycoprotein IIIa. So, we tested the binding of peptides from the 25-42 region of the GPIIIa to purified HLA-DR3 and -DRw52a molecules, using a solid phase assay and a liquid phase peptide binding assay. No binding was demonstrated, indicating that either the crucial region for binding to class II molecules is not the 25-42 region, or that other events only occurring "in vivo" are required for binding. These results may also suggest an indirect role of the residue 33 for T-cell stimulation.

A europium fluoroimmunoassay for measuring binding of antigen to class II MHC glycoproteins

A dissociation-enhanced lanthanide fluoroimmunoassay employing europium-streptavidin and time-resolved fluorimetry was developed to measure binding of biotin-labeled peptides to class II MHC proteins. Binding of biotin-peptides as measured by this assay was saturable and inhibited in the presence of unlabeled peptide. Background fluorescence was minimal and there was a direct relationship between signal and biotin-peptide/class II complex concentration from 1.3 pmol to less than 1 fmol total class II. The sensitivity of the assay and the ability to selectively capture specific class II proteins from detergent lysates of cells with solid phase mAb made it possible to measure formation peptide/class II complexes in live APC cultured with biotin-labeled insulin. This assay is expected to be useful for routine measurement of peptide/class II binding and biochemical analysis of Ag processing events.

Ultrastructural studies bearing on the mechanism of UVB-impaired induction of contact hypersensitivity to DNCB in man

In both murine and human experimental systems, acute, low dose exposure of skin to ultraviolet B light (UVB) impairs the induction of allergic contact dermatitis (ACD) by haptens such as dinitrochlorobenzene (DNCB) in a significant proportion of individuals. By light microscopy, epidermal Langerhans cells (LC) have been reported to be depleted by UVB exposure as well as by epicutaneous hapten application, implying that LC may be the locus of action of the effects of both UVB and DNCB. However, light microscopy can not readily distinguish cell density changes secondary to LC necrosis from changes resulting from down-modulation of expression of LC surface molecules. Using a highly sensitive immunogold electron microscopic approach, we have evaluated the differential effects of UVB and/or DNCB on human epidermal LC. The results reveal that DNCB alone caused significant up-regulation of cell surface HLA class II expression on a very small number of LC, the major fraction of LC expressing normal levels of HLA class II. Furthermore, DNCB alone caused a modest reduction in the density of LC at the treated sites without evidence of cell necrosis. Treatment with UVB alone or UVB exposure followed by DNCB resulted in a reduction in the density of LC, with widespread evidence of LC necrosis. However, the few remaining intact LC were all intensely HLA class II-positive after UVB exposure followed by DNCB, whereas treatment with UVB alone did not result in changes in LC HLA class II expression. The findings that after DNCB painting only a small proportion of the LC were strongly HLA class II-positive, but after UVB exposure followed by DNCB all intact LC displayed significant up-regulation of cell surface HLA class II expression, imply that UVB exposure inhibits the migration of epidermal LC. This is consistent with the view that DNCB fails to induce ACD when hapten is painted on UVB-exposed skin because insufficient LC are available to initiate T cell activation in the draining lymph node.

MHC class I/peptide interactions: binding specificity and kinetics

Recent developments in the preparation of soluble analogues of the major histocompatibility complex (MHC) class I molecules as well as in the application of real time biosensor technology have permitted the direct analysis of the binding of MHC class I molecules to antigenic peptides. Using synthetic peptide analogues with cysteine substitutions at appropriate positions, peptides can be immobilized on a dextran-modified gold biosensor surface with a specific spatial orientation. A full set of such substituted peptides (known as 'pepsicles', as they are peptides on a stick) representing antigenic or self peptides can be used in the functional mapping of the MHC class I peptide binding site. Scans of sets of peptide analogues reveal that some amino acid side chains of the peptide are critical to stable binding to the MHC molecule, while others are not. This is consistent with functional experiments using substituted peptides and three-dimensional molecular models of MHC/peptide complexes. Detailed analysis of the kinetic dissociation rates (kd) of the MHC molecules from the specifically coupled solid phase peptides reveals that the stability of the complex is a function of the particular peptide, its coupling position, and the MHC molecule. Measured kd values for antigenic peptide/class I interactions at 25 degrees C are in the range of ca 10(-4)-10(-6)/s. Biosensor methodology for the analysis of the binding of MHC class I molecules to solid-phase peptides using real time surface plasmon resonance offers a rational approach to the general analysis of protein/peptide interactions.

Functional analysis of the antigen binding region of an MHC class II molecule

The MHC class II molecules bind antigenic peptides and present them to T cells. Their ability to carry out these functions depends, in a critical way, on the detailed structure of the membrane-distal alpha 1 and beta 1 domains of these molecules. Using the I-Ak molecule and a series of hen egg lysozyme (HEL) peptide-specific, I-Ak-restricted T cell hybridomas as a model, we have examined the effect of altering essentially all of the polymorphic residues of the murine class II molecule on its ability to present Ag. Our results support the following conclusions: (1) both the location and the structural alteration introduced in a specific amino acid interchange are important in determining the effect the interchange will have on Ag presentation; and (2) changes in amino acids in the floor of the putative Ag binding cleft of the class II molecule can exert a major influence on the presentation of peptides to T cells. By carrying out direct binding experiments between the HEL(46-61) peptide and two mutant I-A molecules that fail to present HEL(46-61) to appropriate T cells, we were able to assess, in a quantitative fashion, the role played by peptide binding in the failure to present Ag. Our results suggest that, in the two cases studied, the failure to bind the HEL(46-61) peptide was not primarily responsible for the failure of the mutant class II molecule to present that peptide. Specifically, an A beta chain mutant that possesses d allelic residues at positions 65-67 in the second PMR of the Ak beta chain actually binds HEL(46-61) at wild type (I-Ak) levels. In contrast, an A alpha chain chimera in which b allelic residues are inserted in the third PMR of the Ak alpha chain, binds HEL(46-61) about three- to four-fold less well than wild type. While this decrease in binding affinity may be partially responsible for the inability of the latter chimeric molecule to present HEL(46-61), it can not be the total explanation because increasing the peptide concn even by an order of magnitude does not restore Ag presentation by APC expressing this chimeric molecule. These results are discussed in terms of the currently accepted model of the class II molecule.

Identification of two distinct properties of class II major histocompatibility complex-associated peptides

We have examined the interactions of various peptides with the mouse class II major histocompatibility complex molecule I-Ak. The peptides were derived from the model protein hen egg white lysozyme (HEL). The immunodominant peptide of HEL is a 10-mer, residues 52-61. Our previous work established that this sequence contains the key residues for binding and presentation to T cells. Now we show that the binding of this 10-mer sequence resulted in complexes of I-Ak and peptide that, in SDS/PAGE (without boiling the protein), rapidly dissociated from the component alpha and beta chains. The binding interactions were studied in vitro, by incubating purified I-Ak and radiolabeled peptide, or ex vivo, by using antigen-presenting cells incubated with peptides. Peptides with additional residues at either the amino or carboxyl terminus behaved dramatically differently. Complexes of I-Ak with the longer peptides were stable to SDS/PAGE. Very few amino acid additions result in the change from unstable to stable complexes. The important issue here is that when cultured with HEL, antigen-presenting cells selected the HEL peptides containing the 52-61 sequences that favored stability [Nelson, C. A., Roof, R. W., McCourt, D. W. & Unanue, E. R. (1992) Proc. Natl., Acad. Sci. USA 89, 7380-7383]. Also, from other studies, such sequences correlate with a high immunogenicity of the peptide. We conclude that there are structural features of peptides that change the stability of the class II molecule and that are independent of the "core" peptide seen by the T cells.

Position 71 in the alpha helix of the DR beta domain is predicted to influence peptide binding and plays a central role in allorecognition

Despite all the structural and functional data that have been accumulated regarding major histocompatibility complex (MHC) class II molecules during recent years, the relative contribution of putative T cell receptor (TcR)-contacting residues and peptide-binding MHC polymorphisms to MHC-restricted and allospecific T cell responses remains a point of contention. Some authors emphasize the importance of direct interaction between the allospecific TcR and polymorphic MHC residues whereas other emphasize the role of naturally processed MHC-bound peptides. We have previously described a new HLA-DRB1 allele: DR BON (DRB1*0103). This gene differs from DRB1*0101 by six base pairs clustered in the third variable region of the second exon leading to three amino acid changes at positions 67, 70 and 71 of the beta chain of the HLA-DR molecule. To define the respective role of these residues in allorecognition, we have performed site-directed mutagenesis on the DRB1*0103 allele to create six mutants which are intermediary between the DR BON and the DR1 alleles. These mutant cDNA were expressed in mouse fibroblasts and the transfectants with the highest expression of class II molecules were used as stimulators for a panel of ten anti-DR BON and five anti-DR1 alloreactive T cell clones. We demonstrate that the residue at the peptide-binding position 71 is of paramount importance in the alloresponse of these clones. In addition some clones were sensitive to amino acid substitution at the TcR-contacting position 70, while substitution at position 67 affects very few clones. The dominance of residue 71 was also observed with an influenza hemagglutinin-specific HLA-DR BON-restricted T cell line.

Role of flanking variable sequences in antigenicity of consensus regions of HIV gp120 for recognition by specific human T helper clones

Human T helper cells can discriminate among strain variants of HIV gp120 because of T cell clones recognizing non-conserved regions, as demonstrated with T cells from HIV-infected individuals and vaccinated volunteers and with primary T cell lines and clones obtained by in vitro immunization. To obtain a better definition of cross-reactions among T cell determinants within HIV gp120 variants, we used a panel of analog peptides within residues 236-251 from the BRU, MN, SF2 and RF strain sequences to induce primary human T cell lines and clones. Different patterns of response were obtained using each of the analog peptides, although they all share the consensus sequence 246-251. Clones recognizing this sequence were generated by priming with the BRU and RF analog peptides, but not with the SF2 analog peptide. SF2 did not induce any 242-245-specific clones, but only T cells recognizing the 236-240 sequence. A preferential response to residues 236-240 was obtained by priming with the BRU and SF2 peptides, but not with the MN and RF peptides. These results suggest that although the analog peptides exhibit a high degree of homology and share a consensus of the C-terminal sequence (246-251), the T cell response to the conserved sequence 246-251 is controlled by flanking sequences. Therefore the presence of a shared sequence per se does not imply in vitro expansion of clones with that fine specificity, even though such clones are available within the naive repertoire and can be triggered by an analog peptide.

Comprehensive delineation of antigenic and immunogenic properties of peptides derived from the nef HIV-1 regulatory protein

The Human Immunodeficiency Virus (HIV-1) nef regulatory protein, a protein involved in AIDS pathology, was used as a model to investigate and analyze B- and T-cell epitopes. In this paper, we describe the potential structural basis of antigenic and immunogenic reactivity of synthetic peptides derived from the macromolecular antigen. The relationship between B- and T-cell determinants in the context of regulatory mechanisms involved in immune recognition, while integrating recent data concerning MHC presentation. As a result of the recent progress in the field of peptide recognition and presentation, the potential of the peptide approach for constructing successful synthetic vaccines needs to be continuously re-evaluated.

Peptide-MHC interaction in autoimmunity

Our increased understanding of the molecular basis of autoimmunity owes much to an appreciation of general principles governing peptide-MHC interactions. Such understanding may help resolve long-standing questions concerning autoimmune diseases and aid development of improved therapeutic strategies for their treatment.

Antigen-specific, MHC-unrestricted T cells

The published record suggests that in the majority of cases the antigen is recognized by the T cell receptor (TCR) as a complex of a foreign antigen and amino acid residues contributed by the major histocompatibility complex (MHC) antigens, and the antigen-specific, MHC-restricted effector function is an unambiguous result of this process. Alternatively, the T cell receptor may recognize a particular conformational form of the antigen which is dictated by the allelic differences in the MHC, resulting also in MHC-restricted recognition. When, however, a T cell which phenotypically fulfills all the requirements necessary to perform antigen specific, MHC-restricted function, shows a lack of MHC restriction, there are two possible explanations: 1) In addition to the MHC-restricted, antigen-specific T cell receptor the cell expresses, or has newly acquired the expression of another, MHC-unrestricted (NK-like) receptor, or 2) The specific antigen recognized by the T cell receptor, is able to bind to the receptor and activate the T cell without being presented by the MHC molecule. While the first possibility has been extensively described in the literature as well as other articles in this issue, the second possibility has not been dealt with to the same extent and is the primary focus of this review.

Identification of the naturally processed form of hen egg white lysozyme bound to the murine major histocompatibility complex class II molecule I-Ak

A murine B-cell lymphoma bearing the class II major histocompatibility complex molecule I-Ak was cultured with the protein antigen hen egg white lysozyme (HEL). The I-Ak molecules were purified, and their associated peptides were extracted for characterization. Five HEL peptides were identified. Four contained the 10 amino acid residues HEL 52-61 (DYGILQINSR) but were heterogeneous in length and flanking residues. This core sequence is known to confer a high binding affinity for I-Ak. One additional peptide contained the amino acid residues HEL 48-60. These data demonstrate that the HEL epitope containing residues 52-61 is the most abundant HEL epitope presented on the major histocompatibility complex of the antigen-presenting cells and consequently explains its immunodominance.

A polyalanine peptide with only five native myelin basic protein residues induces autoimmune encephalomyelitis

The minimum structural requirements for peptide interactions with major histocompatibility complex (MHC) class II molecules and with T cell receptors (TCRs) were examined. In this report we show that substituting alanines at all but five amino acids in the myelin basic protein (MBP) peptide Ac1-11 does not alter its ability to bind A alpha uA beta u (MHC class II molecules), to stimulate specific T cells and, surprisingly, to induce experimental autoimmune encephalomyelitis (EAE) in (PL/J x SJL/J)F1 mice. Most other amino acid side chains in the Ac1-11 peptide are essentially irrelevant for T cell stimulation and for disease induction. Further analysis revealed that binding to A alpha uA beta u occurred with a peptide that consists mainly of alanines and only three of the original residues of Ac1-11. Moreover, when used as a coimmunogen with MBP Ac1-11, this peptide inhibited EAE. The finding that a specific in vivo response can be generated by a peptide containing only five native residues provides evidence that disease-inducing TCRs recognize only a very short sequence of the MHC-bound peptide.

Peptide-induced proliferation and lymphokine production in human T cells in the absence of antigen-presenting cells: role of T-cell activation state and costimulatory signals

The role of T-lymphocytes as antigen-presenting cells (APCs) for other T cells was investigated. Activated rabies-virus-specific human T-cell clones were shown to present peptide to class II major histocompatibility complex (MHC)-restricted T cells of a different fine specificity, resulting in lymphokine production and cell proliferation. Furthermore, purified and activated antigen-specific T cells could produce lymphokines and proliferate as a result of the addition of antigenic peptide in the absence of APC. The functional response of T cells to peptide in the absence of APC was amplified by the addition of phorbol ester (PMA) and was inhibited with antibodies specific to class II MHC or to the CD2 molecule. Experiments performed in single-cell suspension cultures using semisolid medium prepared with 1% agar demonstrate that T-cell proliferative and lymphokine responses to peptide both in the presence and absence of APC require the interaction of T-cell antigen receptor (TCR) molecules with class II MHC-peptide complexes on different cell surfaces (cell-cell contact). On the other hand, peptide self-presentation, which occurs by the binding of TCR with class II MHC-peptide complexes on the same cell surface (at the single-cell level), resulted in T-cell activation (i.e., high expression of surface CD2, CD25, and HLA-DR molecules), without proliferation or lymphokine secretion, a pattern observed in the induction of T-cell anergy by antigen. The results are discussed in terms of the role of class II MHC molecules on activated T-lymphocytes, which enable these cells to function as "professional APC" in the development of T-cell regulatory networks.

Measles virus transmembrane fusion protein synthesized de novo or presented in immunostimulating complexes is endogenously processed for HLA class I- and class II-restricted cytotoxic T cell recognition

The routes used by antigen-presenting cells (APC) to convert the transmembrane fusion glycoprotein (F) of measles virus (MV) to HLA class I and class II presentable peptides have been examined, using cloned cytotoxic T lymphocytes in functional assays. Presentation by Epstein-Barr virus-transformed B lymphoblastoid cell lines was achieved using live virus, ultraviolet light-inactivated virus, and purified MV-F delivered either as such or incorporated in immunostimulating complexes (MV-F-ISCOM). Only live virus and MV-F-ISCOM allow presentation by class I molecules, while all antigen preparations permit class II-restricted presentation. We observe presentation of MV-F from live virus and as MV-F-ISCOM by class II molecules in a fashion that is not perturbed by chloroquine. Our studies visualize novel presentation pathways of type I transmembrane proteins.

B-cell factor 1 is required for optimal expression of the DRA promoter in B cells

The X box in the DRA promoter of the human histocompatibility complex is required for expression of the DRA gene in B cells. We show that a B-cell factor binds to a sequence that is clearly distinguishable from binding sites for the previously described X box binding nuclear proteins RF-X, NF-X, NF-Xc, NF-S, hXBP, and AP-1. Mutations in the DRA X box that disrupt the binding of this factor result in a lower level of gene expression, as does the presence of Id (a trans-dominant regulatory protein that negatively regulates helix-loop-helix proteins). Furthermore, this factor is recognized by antibodies directed against the helix-loop-helix protein A1, a mouse homolog of the immunoglobulin enhancer binding proteins E12/E47, and it binds to sequences in other genes that were previously shown to bind these proteins. By these criteria, this factor is BCF-1.

B-cell factor 1 is required for optimal expression of the DRA promoter in B cells

The X box in the DRA promoter of the human histocompatibility complex is required for expression of the DRA gene in B cells. We show that a B-cell factor binds to a sequence that is clearly distinguishable from binding sites for the previously described X box binding nuclear proteins RF-X, NF-X, NF-Xc, NF-S, hXBP, and AP-1. Mutations in the DRA X box that disrupt the binding of this factor result in a lower level of gene expression, as does the presence of Id (a trans-dominant regulatory protein that negatively regulates helix-loop-helix proteins). Furthermore, this factor is recognized by antibodies directed against the helix-loop-helix protein A1, a mouse homolog of the immunoglobulin enhancer binding proteins E12/E47, and it binds to sequences in other genes that were previously shown to bind these proteins. By these criteria, this factor is BCF-1.

Antigen processing and presentation: close encounters in the endocytic pathway

Stimulation of helper T cells by class II molecules occurs when the class II molecules bind and display peptides derived from foreign antigens that have been endocytosed. The formation of peptide-class II complexes requires antigen degradation and exposure of the peptide-binding site of class II molecules, both of which depend on proteolysis and low pH in the endocytic pathway. This review discusses the role of specific compartments of the endocytic pathway in the generation of antigenic peptides, and in the binding of antigenic peptides to newly synthesized class II molecules and those that are internalized from the cell surface.

The human class II MHC protein HLA-DR1 assembles as empty alpha beta heterodimers in the absence of antigenic peptide

We have produced the human class II histocompatibility protein, HLA-DR1, as a soluble, secreted glycoprotein in insect cells infected with baculoviruses carrying truncated alpha and beta subunit genes. The peptide-binding site is empty, and the empty molecules are fully competent to bind antigenic peptide. We used the empty molecules to measure an intrinsic rate for peptide association, and to investigate the role of peptide in stabilizing the class II structure. Peptide binding kinetics for the empty molecule are only 10-fold faster than for peptide exchange into an occupied site, suggesting that a conformational change may accompany peptide binding. The native alpha beta heterodimer assembles in the absence of antigenic peptide, but peptide binding stabilizes the empty heterodimer against aggregation and against SDS-induced denaturation.

Solution binding of an antigenic peptide to a major histocompatibility complex class I molecule and the role of beta 2-microglobulin

The major histocompatibility complex-encoded class I molecule, a noncovalent dimer of a polymorphic 45-kDa heavy chain and a nonpolymorphic 12-kDa beta 2-microglobulin (beta 2m) light chain, binds peptide antigen prior to its interaction with T-cell antigen receptors. We report here that the binding in aqueous solution at 37 degrees C of a soluble purified murine major histocompatibility complex class I protein, H-2Lds (a soluble analogue of H-2Ld consisting of the alpha 1 and alpha 2 domains of H-2Ld, the alpha 3 domain and the C terminus of Q10b), to an antigenic peptide is controlled by the light-chain subunit beta 2m. Analysis of the equilibrium binding data favors a model in which two classes of peptide binding sites exist, the high-affinity class having an equilibrium constant for dissociation, KH, of 3.7 x 10(-7) M and accounting for 12% of the theoretically available sites. Studies of binding in the presence of excess beta 2m indicate that this increases the concentration of available high-affinity sites. These data are consistent with a ternary model in which high-affinity sites are generated by the interaction of beta 2m with the peptide-binding class I heavy chain.

Modulation of T cell responses with MHC-derived peptides

T cells are activated by an interaction of their TCRs with a complex made up of antigenic peptide bound to the interhelical groove of MHC molecules. The helices lining the antigen binding groove of MHC molecules are felt to contribute several contact residues for TCR binding. Peptides derived from the amino acid sequences of these helices may be capable of modulating immune responses and aiding in the dissection of immune recognition. These studies address the effects of a peptide derived from the sequence of amino acids 68-83 of the IAk beta 1 domain (IAk 68-83) predicted to represent a portion of an antigen-binding helix on the IAk molecule. The IAk 68-83 peptide is bound by a monoclonal anti-IAk antibody and inhibits its binding to IAk-bearing cells. The IAk 68-83 peptide inhibits antigen-dependent activation of the IAk+con-albumin restricted T cell clone D10.G4, and this effect is more pronounced at lower doses of antigen-presenting cells. The free peptide has a small effect in limiting binding of anticlonotypic antibodies to D10.G4, and a multivalent form bound to BSA has a more pronounced effect in this regard. The BSA-peptide conjugate, when fluoresceinated, specifically stained D10.G4 cells, and this was specifically competed by unfluoresceinated IAk 68-83 peptide-BSA conjugate, as well as by anticlonotype. These results suggest that peptides derived from the predicted helical region of MHC class II molecules may have a direct interaction with T cell receptors. Such peptides may be capable of modulating immune responses in a physiologically significant manner.

A case for chaperones in antigen processing

The assembly of peptide-MHC-class-II molecule complexes by antigen-presenting cells is far more efficient than would be predicted from studies of peptide binding to purified MHC class II molecules in vitro. One possible explanation for this discrepancy is that proteins in the antigen-presenting cell facilitate the assembly process. Here, Diane DeNagel and Susan Pierce present the case for involvement of members of the chaperone/heat shock protein 70 family in the intracellular assembly of processed-antigen-MHC-class-II-molecule complexes.

Effects of localized HLA class II beta chain polymorphism on binding of antigenic peptide and stimulation of T cells

The relationship between HLA-DR1 polymorphism and recognition of antigen by T cells was investigated. Two allelic variants of HLA-DR1, which differ by amino acid substitution at positions 85 and 86 of the beta chain, were characterized for the effect of substitution on recognition of foreign antigen by DR1-restricted T cells. Substitution of alanine and valine for valine and glycine residues at positions 85 and 86 of the DR1 beta chain resulted in deficient T-cell stimulation as demonstrated by the requirement for higher concentrations of antigen to induce maximal levels of T-cell proliferation, induction of lower levels of proliferation at optimal antigen concentrations, and slower kinetics of formation of stimulatory peptide-DR1 complexes. Direct binding studies employing both biotinylated and radioiodinated forms of antigenic peptide demonstrated quantitatively lower levels of peptide bound to substituted DR1 molecules and low levels of site-specific binding as assessed by competitive inhibition analyses. The effect of MHC class II polymorphism on peptide-binding affinity as opposed to induction of appropriate peptide conformation and the impact of polymorphism at DR1 beta chain positions 85 and 86 on allorecognition of HLA-DR1 are discussed.

Biochemical evidence for the rapid assembly and disassembly of processed antigen-major histocompatibility complex class II complexes in acidic vesicles of B cells

Helper T cell recognition of antigen requires that it be processed within antigen-presenting cells (APC) to peptide fragments that subsequently bind to major histocompatibility complex (MHC) class II molecules and are displayed on the APC surface. Heretofore, processed antigen-MHC class II complexes have been detected by functional assays, measuring the activation of specific T cells. We now report direct, biochemical evidence for the assembly of processed antigen-MHC class II complexes within splenic B cells as APC. The I-Ek MHC class II molecules were immunoprecipitated from B cells that had processed the model protein antigen cytochrome c radiolabeled across its entire length by reductive methylation of lysine residues and covalently coupled to Ig-specific antibodies, allowing internalization after binding to surface Ig. Our previous studies showed that I-Ek immunoaffinity purified from B cells that had processed cytochrome c contains functional processed antigen--MHC class II complexes and that approximately 0.2% of the I-Ek molecules are specifically associated with one of two predominant processed antigenic fragments. Here we show that these complexes are rapidly assembled, within 30-60 min after antigen binding to surface Ig on splenic B cells. Maximal numbers of complexes are assembled by 2 h in a process that is sensitive to acidic vesicle inhibitors but not to inhibitors of protein synthesis. The processed antigen-I-Ek complexes have a relatively short half-life of 2-4 h and are disassembled or degraded within 8 h after antigen is first internalized. The disassembly or degradation of the processed antigen-I-Ek complexes requires acidic vesicle function, and in the presence of an acidic vesicle inhibitor the complexes are long lived. Thus, using a biochemical assay to monitor processed antigen-I-Ek complexes, we find that, in B cells, processed antigen is relatively rapidly associated in acidic vesicles with preexisting MHC class II molecules, and the complexes are disassembled 4-6 h later in processes that also require acid vesicle function.

How MHC class II molecules work: peptide-dependent completion of protein folding

It is now clear that peptides play a key role in stabilizing the structure of MHC class II molecules. Here, Scheherazade Sadegh-Nasseri and Ron Germain propose that newly synthesized MHC class II, and indeed class I, molecules behave like partially folded proteins, with peptides acting as a surrogate portion of the MHC polypeptide structure that is necessary for completion of conformational maturation.

Binding of a major T cell epitope of mycobacteria to a specific pocket within HLA-DRw17(DR3) molecules

CD4+ T cells recognize antigenic peptides bound to the polymorphic peptide-binding site of major histocompatibility complex (MHC) class II molecules. The polymorphism of this site is thought to dictate which peptides can be bound and thus presented to the T cell receptor. The mycobacterial 65-kDa heat-shock protein (hsp65) peptide 3-13 is an important T cell epitope: it is immunodominant in the mycobacterium-specific T cell response of HLA-DR3+ individuals but, interestingly cannot be recognized in the context of any other HLA-DR molecules. We, therefore, have tested whether the hsp65 epitope p3-13 is selected for T cell recognition in the context of only HLA-DR3 molecules by an unique binding specificity for HLA-DR3. Using biotinylated peptides and EBV-transformed BLCL comprising all known HLA class II specificities, we find that p3-13 binds to HLA-DRw17(DR3) but not to any other HLA-DR molecule. Conversely, a control peptide p307-319 influenza hemagglutinin binds to all known HLA-DR molecules but only weakly to HLA-DRw17 and HLA-DR9. Peptide binding could be inhibited by excess unbiotinylated competitor analogue as well as by anti-DR monoclonal antibodies but not by anti-class I-, anti-DP- or anti-DQ monoclonal antibodies. The amino acid sequence of DRw17 molecules differs uniquely at five positions from the other DR beta 1 sequences. Three of these five residues (positions 26, 71 and 74) are potential peptide contacting residues. These residues map closely together in the hypothetical three-dimensional model of the DR molecule and, thus, most probably form a positively charged pocket, critical for the binding of p3-13. Interestingly, p3-13 does not bind to a DR3 variant, the DRw18 molecule. The DRw18 beta 1 chain differs from DRw17 at two major positions, close to or within the DRw17-specific pocket. These substitutions drastically change the structure and charge of the pocket and thus presumably abrogate its ability to bind p3-13.

Synthetic peptides as vaccines

The economics of vaccines has been a major limitation in the commercial research and development of new approaches. This coupled with the natural scientific desire to simplify and define the composition of effective vaccines argues that the future of vaccines lies in novel approaches that will discover effective and less expensive components. Peptides, whether they are chemically synthesized or produced in bacteria, are an attractive possibility. To substitute linear peptides for complex mixtures of proteins would be a major technical advance and would stimulate tremendous commercial interest. However, at the present time I view this approach still unlikely to be of major practical importance. I conclude this because of the complexity of immunological responses to microorganisms. Even though, in some instances, a cytotoxic T-cell response or even the majority of the antibody response to a pathogen can be defined by a short linear peptide, most people believe that multiple effector functions of the immune system should be stimulated in optimal vaccines. For a small cocktail of peptides to reproduce the diversity of responses elicited by a virus, parasite, or bacterium is unlikely. However, I fully realize that remarkable progress has occurred towards understanding the structural requirements necessary to stimulate cellular and humoral immune responses, and peptides have been integral in the development of this field. Also, the success of several research groups in developing effective antiviral vaccines using short linear peptides argues that I might be painting too dark of a picture. As someone who has used this strategy to explore peptide-MHC and peptide-antibody interactions, I am a strong scientific supporter of the approach. In this forum I am purposely cautious in my optimism. As the details of the complex molecular and cellular interactions that control the immune system are elucidated, both the number of strategies and the possible applications of modulating the immune response will increase as well. In addition to protective immunity to pathogens, cancer therapy could be revolutionized if tumor-specific cytotoxic T-cells could be generated routinely. Novel therapeutic approaches to allergy, autoimmunity, and transplantation can be envisioned if the T-lymphocytes responsible for these syndromes could be modulated without total immune suppression. Consequently, I am confident that the experiments described in this chapter will be central to developing exciting new therapeutic and prophylactic compounds, but I am not sure that they will resemble naturally occurring peptides. The one aspect I am confident of is that the capacity of the immune response to protect the organism will continually surprise us.

Tumor-immunotherapy with the use of tumor-antigen-pulsed antigen-presenting cells

Our previous study demonstrated that the administration of antigen-presenting cells (APC) pulsed with tumor cell membrane fraction to naive syngeneic mice results in effective induction of tumor-specific protective immunity in vivo. The present study examined whether tumor-antigen-pulsed APC can produce the inhibitory effect on the growth of tumor cells when administered to tumor-bearing hosts. Naive BALB/c mice were inoculated with viable tumor cells. Five days later, these mice started to receive the relevant tumor-antigen-pulsed APC at 3- to 4-day intervals. The administration of tumor-antigen-pulsed APC induced the rejection or growth inhibition of a growing tumor in approximately half of the recipient mice. Moreover, it was demonstrated that tumor-specific immunity was induced in such tumor-regressed mice. These results indicate that tumor-antigen-pulsed APC are effectively applicable to the tumor-specific immunotherapy.

An optimal viral peptide recognized by CD8+ T cells binds very tightly to the restricting class I major histocompatibility complex protein on intact cells but not to the purified class I protein

CD8+ cytotoxic T lymphocytes recognize cell surface complexes formed by class I major histocompatibility complex (MHC-I) glycoproteins and antigenic peptides. We have identified a peptide nonamer (termed IV9) derived from the human immunodeficiency virus that is over a millionfold more active (at subpicomolar concentrations) than peptide analogues longer or shorter by one or two amino acid residues. Although IV9 does not detectably bind to isolated MHC-I molecules as measured by equilibrium dialysis, we quantitated its specific binding in unaltered form to MHC-I on intact cells. Less than 1% of cell surface MHC-I forms complexes with IV9, which suffices to trigger maximal cytotoxic T-lymphocyte activity. By contrast, a peptide dodecamer that includes the IV9 sequence and is active at micromolar concentrations does not bind to MHC-I on intact cells, raising the possibility that this longer peptide undergoes processing. Using stoichiometrically iodinated IV9 to obviate the ambiguities associated with trace labeling methods, we measured the dissociation kinetics of purified peptide/MHC-I complexes isolated by affinity chromatography and found these complexes to be exceedingly stable (t1/2 = 200-600 hr).

Reactions of the subunits of the class II major histocompatibility complex molecule IAd

Major histocompatibility complex (MHC) class II molecules are heterodimers formed by noncovalent linkage of alpha and beta chains. It has been shown that the subunits of the MHC class II molecules IAd and IEk bind antigenic peptides as well as antigenic peptides labeled with fluorescent probes. Laser scanning fluorescence microscopy on SDS/polyacrylamide gels demonstrates that the subunit-peptide complexes of IAd are stable over a wide pH range. Below pH 5.3 the heterodimer of IAd dissociates into the free chains, which still bind antigenic peptides such as the 18-amino acid peptide obtained by a tyrosine addition to a chicken ovalbumin peptide, Ova-(323-339)Y. The stability of preformed subunit complexes with fluorescein-labeled Ova-(323-339)Y was investigated by using high-performance size exclusion chromatography and epifluorescence microscopy. Each subunit forms a long-lived complex, both in detergent solutions and in reconstituted lipid bilayers. At 37 degrees C and pH 7.0 the dissociation half-time of the beta-subunit-peptide complex was determined to be 28 hr and that of the alpha-subunit-peptide complex was 10 hr. In contrast to the dissociation of the peptide from the IAd heterodimer, the half-times for dissociation of the peptide from the separate chains are not decreased at pH 5.0.

Reduction of disulfide bonds during antigen processing: evidence from a thiol-dependent insulin determinant

Previous studies have demonstrated that insulin, like other protein antigens, requires processing in metabolically active antigen-presenting cells (APC) before it can be recognized by class II-restricted T lymphocytes. Unlike many other proteins, insulin peptides of minimal size retain the requirement for antigen processing. We demonstrate that this requirement can be bypassed by incubation of insulin with reducing agents in the presence of aldehyde-fixed APC. Fixed APC treated in this way were able to stimulate I-Ab- and I-Ad-restricted T cell hybridomas. Data are presented that demonstrate that cloned and polyclonal T cells recognize a determinant within the NH2-terminal 14 residues of the beef insulin A chain with no requirement for B chain residues. The common feature among peptides capable of stimulating these cells in the presence of live APC is the chemical form of the cysteine thiol groups. Those forms that produce free thiols upon reduction are active, whereas those with irreversibly protected sulfhydryls are not. Functional experiments with fixed APC and competition binding experiments with purified I-Ad indicate that only A chain peptides with free thiols are able to stably associate with the peptide-binding site on class II in a form that is recognized by specific T cells. Our findings indicate that reduction of disulfide bonds is both necessary and sufficient for presentation of insulin to a major population of class II-restricted T cells. The results provide strong support for the hypothesis that protein disulfides can be reduced during physiologic antigen processing.

Enhanced binding of peptide antigen to purified class II major histocompatibility glycoproteins at acidic pH

Helper T lymphocytes recognize peptide antigens stably associated with class II major histocompatibility complex (MHC) glycoproteins on the surface of antigen-presenting cells and serve to regulate a wide variety of immune responses. A previous study from our laboratory had demonstrated that the functional association of various peptide antigens with the antigen-presenting cell membrane was increased at pH 5 as compared to pH 7, consistent with the potential role of acidic endosomal compartments in antigen processing. The mechanism for this effect was not determined. In the present study, assays using purified class II glycoprotein were used to further define this mechanism. The potential requirement for pH-dependent interactions involving non-MHC membrane components was excluded in functional assays with purified class II reconstituted in artificial membranes containing only neutral phospholipids and cholesterol. The association of HEL(104-120) with I-Ed, and OVA(323-339) with I-Ad, was increased at pH 5, as measured by activation of specific T cell hybridomas. An enzyme immunoassay was developed to measure the binding of biotin-labeled peptides to purified class II in detergent micelles. The pH dependence of binding paralleled our previous functional results. Optimum binding of biotin-HEL(104-120) to I-Ed was observed at pH approximately 4.5, whereas maximum binding of biotin-Myo(106-118) to I-Ad occurred at pH approximately 5.5. The latter peptide also bound weakly to I-Ed, but with a pH dependence similar to that observed using HEL(104-120). Further experiments with biotin-HEL(104-120)/I-Ed indicated that both the apparent affinity and the apparent concentration of peptide-binding sites are increased as hydrogen ion concentration is increased from pH 7 to pH 5. The effect of pH in this range was largely reversible and was not associated with a change in peptide dissociation that could be measured with our assay system. Binding was not inhibited in the presence of 1.5 M NaCl, suggesting that electrostatic interactions between HEL(104-120) and I-Ed are not essential for binding. It is proposed that protonation of a critical group(s) in the class II molecule regulates its capacity to form stable complexes with peptide. However, this effect alone does not fully account for the rapid kinetics of peptide binding observed in experiments with intact antigen-presenting cells.

HLA-DR and -DQ allelic sequences in multiple sclerosis patients are identical to those found in the general population

The HLA-DR2/Dw2 haplotype is associated with multiple sclerosis (MS) in the North American Caucasian population. HLA-DRB, -DQA, and -DQB N-terminal domain sequences derived from amplified cDNA in a series of North American Caucasian MS patients were examined to determine if unique or rare class II alleles could be found. In addition, class II allelic sequences were analyzed from clinically discordant, HLA-genoidentical siblings from a multiplex MS family. All alleles observed, whether from HLA-DR2/Dw2 positive or negative individuals, were identical to those most commonly expressed in the general population. These data demonstrate that, if HLA class II truly confers susceptibility to MS, commonly expressed alleles are involved.

Sequence analysis of peptides bound to MHC class II molecules

CD4 T cells recognize peptide fragments of foreign proteins bound to self class II molecules of the major histocompatibility complex (MHC). Naturally processed peptide fragments bound to MHC class II molecules are peptides of 13-17 amino acids which appear to be precessively truncated from the carboxy terminus, perhaps after binding to the MHC class II molecule. The finding of predominant self peptides has interesting implications for antigen processing and self-non-self discrimination.

Interleukin 10 (IL-10) and viral IL-10 strongly reduce antigen-specific human T cell proliferation by diminishing the antigen-presenting capacity of monocytes via downregulation of class II major histocompatibility complex expression

Interleukin 10 (IL-10) and viral IL-10 (v-IL-10) strongly reduced antigen-specific proliferation of human T cells and CD4+ T cell clones when monocytes were used as antigen-presenting cells. In contrast, IL-10 and v-IL-10 did not affect the proliferative responses to antigens presented by autologous Epstein-Barr virus-lymphoblastoid cell line (EBV-LCL). Inhibition of antigen-specific T cell responses was associated with downregulation of constitutive, as well as interferon gamma- or IL-4-induced, class II MHC expression on monocytes by IL-10 and v-IL-10, resulting in the reduction in antigen-presenting capacity of these cells. In contrast, IL-10 and v-IL-10 had no effect on class II major histocompatibility complex (MHC) expression on EBV-LCL. The reduced antigen-presenting capacity of monocytes correlated with a decreased capacity to mobilize intracellular Ca2+ in the responder T cell clones. The diminished antigen-presenting capacities of monocytes were not due to inhibitory effects of IL-10 and v-IL-10 on antigen processing, since the proliferative T cell responses to antigenic peptides, which did not require processing, were equally well inhibited. Furthermore, the inhibitory effects of IL-10 and v-IL-10 on antigen-specific proliferative T cell responses could not be neutralized by exogenous IL-2 or IL-4. Although IL-10 and v-IL-10 suppressed IL-1 alpha, IL-1 beta, tumor necrosis factor alpha (TNF-alpha), and IL-6 production by monocytes, it was excluded that these cytokines played a role in antigen-specific T cell proliferation, since normal antigen-specific responses were observed in the presence of neutralizing anti-IL-1, -IL-6, and -TNF-alpha mAbs. Furthermore, addition of saturating concentrations of IL-1 alpha, IL-1 beta, IL-6, and TNF-alpha to the cultures had no effect on the reduced proliferative T cell responses in the presence of IL-10, or v-IL-10. Collectively, our data indicate that IL-10 and v-IL-10 can completely prevent antigen-specific T cell proliferation by inhibition of the antigen-presenting capacity of monocytes through downregulation of class II MHC antigens on monocytes.

A first-order reaction controls the binding of antigenic peptides to major histocompatibility complex class II molecules

Major histocompatibility complex class II molecules have been reported to bind antigenic peptides very slowly in vitro. To investigate the molecular events that govern the slow binding reaction, we have determined the dependence of complex formation and dissociation on peptide concentration. The complex between the purified major histocompatibility complex class II protein I-Ek and a fluoresceinated peptide representing amino acids 89-104 of pigeon cytochrome c (FpCytc) was studied. Two important results emerge from this study. (i) At pH 5.4, the half-time for I-Ek-FpCytc complex formation is equal to approximately 7 hr for peptide concentrations that vary over a range of three orders of magnitude. There is in fact a small but significant decrease in the half-time for complex formation at low peptide concentrations. The small decrease in half-time is related to the release of endogenous peptides. (ii) At large ratios of peptide to protein [( FpCytc]/[I-Ek] greater than 40), the half-times for I-Ek-FpCytc complex formation and dissociation are equal to one another to within a factor of two between pH 7.5 and 4.5. The percent results demonstrate that a slow, first-order reaction precedes complex formation between I-Ek and FpCytc. This first-order reaction may involve a protein conformational change in addition to the release of endogenous peptides.

Characterization of naturally processed antigen bound to major histocompatibility complex class II molecules

Helper T lymphocytes recognize peptide fragments of antigen bound to major histocompatibility complex (MHC) class II molecules presented on the surface of antigen-presenting cells (APCs). Previous studies showed that the MHC class II, I-Ek molecules purified from APCs that had processed Drosophila melanogaster cytochrome c (DMc) contained functional, processed antigen-I-Ek complexes. This was demonstrated by the ability of purified I-Ek, incorporated into liposomes, to stimulate DMc-specific T cells in the absence of any additional antigen. Here we describe the isolation and characterization of the processed antigen bound to I-Ek. This was accomplished using DMc radiolabeled across its entire length by reductive methylation of its lysine residues, allowing an analysis of the totality of processed antigen bound to MHC class II molecules. After processing, only about 0.2% of the APC I-Ek molecules contained processed DMc (approximately 800 per cell), yet these were sufficient to stimulate specific T cells. The DMc peptides isolated from the I-Ek molecules showed only two predominant radioactive peaks as analyzed by reverse-phase chromatography. Less processed antigen was bound to purified I-Ak molecules, and these peptides were distinct from those bound to I-Ek. The association of processed DMc with the I-Ek and I-Ak molecules appears highly specific in that no radiolabeled peptides were isolated from purified MHC class I molecules, Kk and Dk, or from the B-cell differentiation antigen B220. The majority of processed antigen-I-Ek complexes migrated more slowly than the majority of the I-Ek protein as analyzed by SDS/PAGE under nonreducing conditions without heating of the sample. This form of I-Ek may be analogous to the earlier described "floppy" form of MHC class II molecules [Dormair, K., Rothenhausler, B. & McConnell, H. M. (1990) Cold Spring Harbor Symp. Quant. Biol. 54, 409-416]. Since newly processed antigen binds nearly exclusively to this slow-migrating form, it may be of functional significance.

MHC class II structure, occupancy and surface expression determined by post-endoplasmic reticulum antigen binding

Class II major histocompatibility complex molecules undergo a change in structure upon stable binding of peptide antigen. Analysis of the site and extent of this change among class II molecules of splenic antigen-presenting cells reveals the preference of class II for peptide acquisition outside the endoplasmic reticulum and indicates that the class II presentation system is not saturated with self peptides. There are numerous empty class II molecules on the cell surface and peptide antigen is evidently important in regulating surface class II expression.

A role for peptide in determining MHC class II structure

T lymphocytes recognize antigen-derived peptides associated with major histocompatibility complex (MHC) class I or class II proteins. Peptide is critical in class I heavy-chain folding and/or stable association with beta 2-microglobulin. Although data exist suggesting a relationship between class II structure and peptide association, no equivalent positive contribution of peptide to the folding state or stability of class II dimers has yet been demonstrated. We report here that most purified E alpha k E beta k molecules leaving low pH in the absence of specific peptide lack a compact, stable dimeric structure. Brief exposure to the appropriate peptide just before and during neutralization promotes this specific conformation in proportion to stably bound peptide, indicating that peptide is important in determining class II MHC structure. Our results also indicate that efficient generation of long-lived peptide-class II complexes involves two stages: initial peptide binding in an acidic environment, which enhances the ability of class II to enter a conformation, from which stabilization upon neutralization results in high-affinity binding of previously associated peptide.

The binding affinity and dissociation rates of peptides for class I major histocompatibility complex molecules

Peptides of various lengths derived from the influenza nucleoprotein (NP) bind to H-2Db class I molecules with affinities at 4 degrees C between approximately 3 x 10(5)- approximately 3 x 10(7) M-1. The peptide with the highest affinity corresponds to the sequence of nine amino acids (NP366-374) recently isolated from cells infected with influenza. This peptide forms stable complexes with half-lives greater than 110 h at 4 degrees C, 39 h at 22 degrees C and 3 h at 37 degrees C. Small increases in length of the peptide greatly reduce the stability of the complex (t1/2 approximately 1-10 h at 4 degrees C). These results may explain the homogeneous length of peptides isolated from class I molecules formed in vivo, and suggest that class I and II may differ in their dependence on the length of peptides for the formation of stable complexes.

Human double-negative (CD4-CD8-) T cells bearing alpha beta T cell receptor possess both helper and cytotoxic activities

Expression of CD4 or CD8 on the cell surface is an important guide for discriminating the immunological functions of T cells. However, a minor T cell subset, which lacks both CD4 and CD8 molecules but bears the usual form of T cell receptor (TCR) alpha beta (CD4-CD8-TCR alpha beta+ T cells), has recently been found not only in mice but also in humans, and its role in immune response is now of considerable interest. In order to clarify the characteristics of this newly defined T cell subpopulation, we established five IL-2-dependent CD4-CD8-TCR alpha beta+ T cell clones from the peripheral blood of a healthy individual, and examined their various biological functions. It was found that all clones not only helped B cells in immunoglobulin production, but also exerted major histocompatibility complex-unrestricted cytotoxicity. Although their CD3/TCR complexes were functionally competent, the cytotoxicity seemed to be mediated via unknown molecules other than the CD3/TCR complex, as evidenced by the failure of CD3 MoAb to inhibit the cytotoxic activity. Our present findings showed that CD4-CD8-TCR alpha beta+ T cells possess potential bifunction, i.e. helper and cytotoxic activities. Their roles in the pathogenesis of immunodeficiency are discussed.

Novel methods to rapidly and sensitively analyze antigenic peptide binding to MHC class II molecules

One of the important steps for antigen presentation by MHC class II molecules involves binding of a peptide fragment of the antigen to the class II molecule followed by recognition of the resulting complex by T cells. The most commonly used methods for studying binding of peptide to MHC II are: equilibrium dialysis, gel filtration chromatography, HPLC and polyacrylamide gel electrophoresis. Each of these methods has some limitations and is time consuming. In addition, each requires a considerable amount of native MHC class II, which is always difficult to obtain. In this report, we describe three different sensitive methods using radiolabeled peptide to study peptide binding to murine MHC class II molecules. These are: nitrocellulose filter binding, thin-layer chromatography (TLC) using plate-supported silica gel or PEI cellulose, and paper electrophoresis using Sepraphor cellulose polyacetate paper. All three methods are rapid, highly sensitive and require only ng quantities of affinity pure MHC class II molecules and peptides. These methods can be used to calculate the peptide occupancy of MHC class II molecules.

Identification of a thyroxine-containing self-epitope of thyroglobulin which triggers thyroid autoreactive T cells

Although thyroglobulin (Tg), the thyroid prohormone, is well known as a T cell dependent autoantigen in human and experimental autoimmune thyroid disease, very little is known about the molecular basis of Tg recognition by T cells. In this paper, we have characterized the epitopes recognized by two clonotypically distinct, murine Tg autoreactive T cell hybridomas, CH9 and ADA2. In vitro iodination of a Tg preparation which was deficient in in vivo organified iodine was first used to confirm our previous observation that these T cells recognize iodination-related epitopes in the Tg molecule. Affinity chromatography of tryptic peptides derived from normally iodinated human Tg revealed that these epitopes were exclusively located in thyroxine (T4) containing peptides. Through the use of synthetic T4-containing peptides, representing the four major hormonogenic sites in Tg, we demonstrated that both CH9 and ADA2 recognize an epitope containing the T4 at position 2553 in human Tg. Sets of overlapping 5mer to 12mer peptides around this T4 showed that the most potent peptide was a 9mer beginning at Asp 2551. The T4 was shown to be a critical residue, since its replacement with any of the 20 naturally occurring amino acids produced only nonstimulatory peptides. Since the T cell hybridomas could also be stimulated by major histocompatibility complex class II positive (interferon-gamma-treated) thyroid epithelial cells in vitro, and their parent T cell lines can induce thyroiditis on adoptive transfer, the T4-containing Tg sequence described here is implicated as a pathogenic epitope in murine thyroid autoimmunity.

Assembly and intracellular transport of major histocompatibility complex molecules

Recent advances in our understanding of biosynthesis and assembly of MHC subunits are discussed. Intracellular traffic of MHC proteins is reviewed in the context of antigen presentation. While the overall picture of antigen presentation is now clear, much of the detail of the early stages of assembly of MHC products remains to be established. The degradative pathways that result in the peptides presented by MHC molecules (in particular those serving Class I molecules) have not yet been identified with certainty.

Non-radioactive detection of MHC class II-peptide antigen complexes in the sub-picomole range by high-performance size-exclusion chromatography with fluorescence detection

In order to avoid chemical or structural modification of T-cell epitopes by labelling, a high-performance size-exclusion chromatographic fluorescence binding assay was developed, based on the intrinsic Trp fluorescence of major histocompatibility complex (MHC) proteins. The increase in Trp fluorescence intensity of the isolated human MHC product HLA-DR 1 on complex formation with unlabelled influenza matrix peptide[18-29] (IM[18-29]) was examined. Binding of IM[18-29] to the heterodimeric form of HLA-DR 1 (Kd = 4.8 mM) and to the disassembled alpha-and beta-subunits (Kd = 9.2 mM) could be demonstrated. In addition, the assay showed the peptide-induced formation of a dimeric conformer of HLA-DR 1, the nature of which is still undefined. Detection of HLA-DR 1 subunit-peptide complexes was possible in amounts of 25 ng in 10 microliter (80 fmol/microliter). The technique proved to be reproducible and less time consuming than common methods that need fluorescence or radioactive labelling.