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

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.

Peptide binding to class I MHC on living cells and quantitation of complexes required for CTL lysis

Antigenic peptides are presented to CD8+T lymphocytes by class I major histocompatibility complex (MHC) molecules. Peptides specifically bind to purified class I molecules in vitro, and to class I molecules on cells at nonphysiological temperatures. We report here the kinetic and equilibrium parameters for the binding of radiolabelled influenza nucleoprotein peptides (NP-Y365-380 and shorter homologues) to the murine H-2Db molecule on intact, viable cells at 37 degrees C. In contrast to earlier reports, we show that peptide binding is rapid and reversible, with dissociation constants ranging from nanomolar to micromolar, suggestive of typical ligand-receptor interactions. Only 10% of cell-surface Db molecules can bind these peptides. To address the relationship between peptide binding and T-cell recognition of the antigen-MHC complex, we determined the minimum number of complexes required to sensitize a target cell for lysis by class I-restricted cytotoxic T-lymphocytes. Our data indicate that EL4 thymoma cells (H-2b) can be sensitized for lysis by cytotoxic T-lymphocytes when as few as 200 class I-peptide complexes (less than 0.08% of surface Db molecules) are present per cell.

Antibody recognition of an immunogenic influenza hemagglutinin-human leukocyte antigen class II complex

The A/Japan/57 influenza hemagglutin (HA) peptide HA 128-145, when bound by human histocompatibility leukocyte antigen-DRw11 cells, is recognized by the human CD4+ T cell clone V1. A rabbit antiserum has been raised against HA 128-145 which recognizes not only the free peptide, but also the HA 128-145/DRw11 complex on a solid matrix, in solution, or on the surface of viable cells. The detection of these complexes on viable cells was shown to be class II specific, DRw11 restricted, and commensurate with the level of DRw11 expression. The identity of DRw11 as the cell surface molecule binding HA 128-145 was confirmed by immunoprecipitation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and tryptic peptide mapping. Using this antiserum HA 128-145/DRw11 complexes could be detected on the cell surface as soon as 30 min after the peptide was added, and increased up to 24 h. Dissociation kinetics showed these complexes were long-lived, with a half-life of approximately 14 h. This anti-HA peptide antiserum represents the first direct means of studying antigenic peptide-human leukocyte antigen class II complexes on the surface of living cells without the addition of a non-amino acid moiety to the peptide. The properties of this antiserum thus provide the potential to study naturally processed antigenic peptides as well as the mechanism of processing itself in a physiologically relevant system.

Expression of a class II major histocompatibility complex (MHC) heterodimer in a lipid-linked form with enhanced peptide/soluble MHC complex formation at low pH

A murine class II major histocompatibility complex (MHC) heterodimer, Ek, expressed as a glycan-phosphatidyl inositol-anchored chimera on Chinese Hamster Ovary cells, can present peptides, but not processed antigen to T cells. This chimeric MHC requires a 100-times higher peptide concentration to achieve a two- to four-times lower level of T cell stimulation. Cleavage with phosphatidylinositol-specific phospholipase C and purification result in large quantities of heterodimer in a water-soluble form. Plates coated with this material and then incubated with peptide can efficiently stimulate the appropriate T cell hybridomas. This stimulation is significantly enhanced when peptides are preincubated with the plate-bound MHC molecules in a pH range (5.0-5.5) similar to that of late endosomes. More than half of the soluble Ek molecules can form a specific complex with cytochrome c peptides in this pH range. This suggests that class II MHC molecules undergo distinct conformational changes in endosomal compartments that render them more capable of forming functional complexes with peptide antigens, irrespective of other cell components.

Kinetics of antigenic peptide binding to the class II major histocompatibility molecule I-Ad

Using high-performance size-exclusion chromatography and fluorescence spectroscopy, we investigated the kinetics of fluorescent peptide reactions with detergent-solubilized I-Ad, a class II molecule of the mouse major histocompatibility complex. At pH 7.0 and 37 degrees C the half-time for the binding of a fluorescein-labeled synthetic peptide representing ovalbumin amino acids 323-339 [FOva-(323-339)Y] to I-Ad was 32 hr, independent of added fluorescent peptide concentration in the range 5-200 microM. Peptide exchange experiments were also carried out, where it was found that the half-time of FOva-(323-339)Y binding was equal to the half-time of dissociation of the Texas Red-labeled peptide TROva-(323-339)Y. These experiments show that slow peptide binding to class II major histocompatibility molecules may be limited by the slow dissociation of prebound peptides. Paradoxically, however, this kinetic behavior--a peptide concentration-insensitive on-reaction with a half-time for peptide binding approximately equal to the half-time for dissociation--can be modeled in more than one way. Models involving a kinetic intermediate are particularly attractive. The kinetics were significantly different at pH 5.0. The half-times for peptide binding and dissociation were approximately 7 times shorter than at pH 7.0. In addition the complex of the I-Ad alpha/beta heterodimer with FOva-(323-339)Y was unstable and dissociated into separate alpha and beta chains with a half-time of approximately 7 hr.

Human autologous tumor-specific T cells in malignant melanoma

T cell lines and clones with autologous tumor-specific activity have been developed in malignant melanoma by stimulating peripheral blood lymphocytes (PBL), lymph node lymphocytes or tumor-infiltrating lymphocytes (TIL) with autologous melanoma cells in the presence of recombinant interleukin 2 (rIL2). T-cell lines and clones have been developed with specific cytotoxicity and/or proliferative responses for autologous melanoma targets but not for allogeneic melanoma tumor cells, autologous normal cells or natural killer (NK)-sensitive targets. The concentration of rIL2 is critical for the generation of autologous tumor-specific T-cell lines, with low rIL2 concentrations (up to 800 IU/ml) facilitating the growth of T-cell lines with tumor-specific activity. The alpha beta T-cell receptor (TCR) and the CD3 antigen are involved in specific cytotoxicity and/or proliferative responses of these T-cell lines and clones. An oligoclonal pattern of beta-chain TCR gene rearrangements was observed on T-cell lines and clones with autologous tumor-specific cytotoxicity, suggesting that they are comprised of T cells that have undergone a clonal expansion in response to particular antigen. Autologous tumor-specific cytotoxic T cells are HLA-restricted and recognize on the melanoma tumor cells HLA Class I or possibly Class II antigens plus a tumor-specific determinant. TIL from patients with metastatic melanoma have unique characteristics in comparison with PBL and lymph node lymphocytes and they appear to contain substantial proportions of T cells that have been locally sensitized to autologous tumor cells. Single stimulation of TIL with autologous tumor cells in the presence of rIL2 is sufficient for the generation of T cell lines with autologous tumor-specific activity, whereas, multiple stimulation of PBL and lymph node lymphocytes was required to achieve the same purpose. TIL-derived T cell lines have been expanded in rIL2 in vitro by at least 1,500-fold without losing their activity. Approximately, 40% of the patients exhibited complete or partial responses to adoptive immunotherapy with melanoma TIL and rIL2.

Refined structure of the human histocompatibility antigen HLA-A2 at 2.6 A resolution

The three-dimensional structure of the human histocompatibility antigen HLA-A2 was determined at 3.5 A resolution by a combination of isomorphous replacement and iterative real-space averaging of two crystal forms. The monoclinic crystal form has now been refined by least-squares methods to an R-factor of 0.169 for data from 6 to 2.6 A resolution. A superposition of the structurally similar domains found in the heterodimer, alpha 1 onto alpha 2 and alpha 3 onto beta 2m, as well as the latter pair onto the ancestrally related immunoglobulin constant domain, reveals that differences are mainly in the turn regions. Structural features of the alpha 1 and alpha 2 domains, such as conserved salt-bridges that contribute to stability, specific loops that form contacts with other domains, and the antigen-binding groove formed from two adjacent helical regions on top of an eight-stranded beta-sheet, are analyzed. The interfaces between the domains, especially those between beta 2m and the HLA heavy chain presumably involved in beta 2m exchange and heterodimer assembly, are described in detail. A detailed examination of the binding groove confirms that the solvent-accessible amino acid side-chains that are most polymorphic in mouse and human alleles fill up the central and widest portion of the binding groove, while conserved side-chains are clustered at the narrower ends of the groove. Six pockets or sub-sites in the antigen-binding groove, of diverse shape and composition, appear suited for binding side-chains from antigenic peptides. Three pockets contain predominantly non-polar atoms; but others, especially those at the extreme ends of the groove, have clusters of polar atoms in close proximity to the "extra" electron density in the binding site. A possible role for beta 2m in stabilizing permissible peptide complexes during folding and assembly is presented.

Binding of radioiodinated influenza virus peptides to class I MHC molecules and to other cellular proteins as analyzed by gel filtration and photoaffinity labeling

In order to determine how T cell-presented peptides associate with the antigen binding sites (desetopes) of class I major histocompatibility complex (MHC) molecules and how they might be scavenged from an endogenous processing pathway for transfer to those molecules, we characterized the binding of two synthetic peptides restricted by HLA-B37 or HLA-A2 to class I MHC molecules and to cellular proteins of histotyped cell lines, by gel filtration and photo-affinity labeling techniques. In gel filtration binding studies, each peptide associated with immunopurified class I MHC molecules from cells with its restricting, histotype, but little was bound to class I MHC molecules from cells without the restricting histotype and none was bound to bovine serum albumin. After crosslinkage of a radioiodinated photoreactive derivative of influenza virus nucleoprotein peptide NP(336-355Y) and immunoprecipitations with antibodies to class I MHC molecules, that peptide was found to bind to immunopurified class I MHC molecules from HLA-B37+ but not HLA-B37- cells. Binding of the [125I]NP peptide increased from 6 to 12 hr of incubation and was competed by unlabeled, NP peptide but not by HLA-A2-restricted, influenza virus matrix MA(57-73). The principal microsomal membrane proteins binding [125I]NP were about 65, 45 and 33 kD.

Effects of pH and polysaccharides on peptide binding to class II major histocompatibility complex molecules

The binding of immunogenic peptides to class II major histocompatibility molecules was examined at various pH values. We studied binding of peptides containing residues 52-61 from hen egg lysozyme (HEL) to I-Ak on fixed peritoneal macrophages or to solubilized affinity-purified I-Ak. Optimum binding occurred at pH 5.5-6.0 with accelerated kinetics relative to pH 7.4; equilibrium binding was also higher at pH 5.5-6.0 than at 7.4. Similar enhancement at pH 5-6 was observed for the binding of hemoglobin-(64-76) to I-Ek and of ribonuclease-(41-61) to I-Ak. In contrast, the binding of HEL-(34-45) to I-Ak was minimally enhanced at acid pH. Dissociation of cell-associated or purified peptide-I-Ak complexes was minimal between pH 5.5 and 7.4, with increased dissociation only at or below pH 4.0 [HEL-(46-61)] or pH 5.0 [HEL-(34-45)]. Thus, optimum peptide binding occurs at pH values similar to the endosomal environment, where the complexes appear to be formed during antigen processing. In addition, we examined the effect of a number of polysaccharides on the binding of peptide to I-Ak. None of these competed with the HEL peptide 125I-labeled YE52-61 for binding to I-Ak. [3H]Dextran also failed to bind purified I-Ak. Polysaccharides do not appear to bind to class II major histocompatibility complex molecules, which explains the T-cell independence of polysaccharide antigens.

Antigen-presenting cells constitutively bind tumor antigens in the tumor-bearing state in vivo to construct an effective immunogenic unit

Antigen-presenting cells (APC) constitutively process endogenous (self) proteins to bind the processed peptides to Ia molecules. In the present study, we investigated whether the same associative recognition also holds true for tumor-associated antigens (TAA) that are regarded as "self" molecules in tumor-bearing hosts. The following results were obtained: (i) an APC-depleted splenic T cell population from CSA1M tumor-immunized hosts was stimulated to produce interleukin 2 in vitro when co-cultured with APC from CSA1M-bearing mice, but not from normal mice; and (ii) a Thy-1+ cell-depleted APC population from CSA1M-bearing mice could produce CSA1M tumor-specific protection in vivo when inoculated into naive syngeneic mice. These results provide evidence for the functional binding in vivo of TAA to APC in the tumor-bearing state. The results are discussed in the context of the paradox that tumor-bearers fail to reject their own malignancy despite the formation on APC of an effective immunogenic unit which is capable of stimulating tumor-specific T cells.

Mechanisms of T cell recognition with application to vaccine design

Both helper and cytotoxic T lymphocytes generally recognize protein antigens not in their intact form, as antibodies do, but on the surface of another cell, after "processing" by that cell to unfold or cleave the protein into fragments and after association of the processed antigen with major histocompatibility complex (MHC) molecules on that cell. This complex process leads to immunodominance of certain segments from the protein, which depends not only on structural features intrinsic to the antigenic segment itself, but also on antigen processing and on the structure of the MHC molecules of the responding individual. We have explored all three of these factors, including the enzymes involved in processing, the way peptides bind to MHC molecules, and structural features such as helical amphipathicity that seem to favour T cell recognition. We have used this information to locate and characterize antigenic sites of proteins of interest for vaccine development, including proteins from the malaria parasite and the AIDS virus, HIV. For HIV, we have identified both helper and cytotoxic T cell sites, coupled a helper site to a B cell site to produce a synthetic immunogen that elicits neutralizing antibodies, and studied the effect of viral sequence variation on cytotoxic T cell recognition and binding of the immunodominant peptide to MHC molecules. This information suggests strategies for the rational design of synthetic or recombinant vaccines.

Oxidized low-density lipoprotein reduces plasma coagulation in vitro

Oxidation of low-density lipoprotein (LDL) was shown to occur in vivo and involved lipid peroxidation and apolipoprotein modification. We studied the effect of oxidized-LDL (Ox-LDL) on plasma coagulation by measuring prothrombin time (PT) and partial thromboplastin time (PTT) following the addition of Ox-LDL to normal plasma. Ox-LDL, but not native LDL, caused prolongation of PT and PTT by 30% in a dose- and time-dependent pattern. This effect was also shown to be present following lipoprotein delipidation, suggesting that it was the apolipoprotein fraction of Ox-LDL, but not its lipid fraction, that was responsible for the prolongation of PT and PTT. This was further substantiated since similar effect could be obtained by adding LDL treated with trinitrobenzenesulphonic acid to block the lysine groups, as occurs in oxidized LDL. Ox-LDL, unlike LDL, was found to reduce plasma ionized calcium by 33%. Moreover, adding calcium ions to Ox-LDL negated its effect on PT and PTT, suggesting that Ox-LDL apolipoprotein may influence coagulation by binding calcium ions.

The CD4 molecule is not always required for the T cell response to bacterial enterotoxins

T cells respond in a V beta-restricted fashion to bacterial enterotoxins bound to major histocompatibility complex (MHC) class II molecules. The requirement for CD4 in MHC class II-restricted T cell responses is very well established. We have assessed the role of CD4 in the T cell response to the bacterial enterotoxins Staphylococcal enterotoxin A (SEA), SEB, and toxic shock syndrome toxin 1. Three CD4- murine T cell hybridomas were transfected with the human CD4 molecule and assayed for interleukin 2 production in the presence of accessory cells bearing human MHC class II molecules and of the appropriate enterotoxin. The results clearly indicate that CD4- cells responded even to suboptimal concentrations of enterotoxin(s) equally well as CD4+ cells. Furthermore, expression of CD4 did not result in the acquisition of previously undetectable reactivity to enterotoxins. These results suggest that unlike the case with antigen-specific responses, formation of a T cell receptor-CD3/CD4 supramolecular complex is not always essential for T cell activation by bacterial enterotoxins.

The cell biology of antigen processing

T lymphocytes recognize antigen only after a series of intracellular events known as antigen processing. The result of antigen processing is the production of short segments of the primary peptide sequence bound to a polypeptide-binding groove on major histocompatibility complex (MHC) molecules. Antigen originates from one of two sites: intracellular or extracellular. There are two corresponding pathways for antigen processing and two corresponding classes of MHC molecule. Analysis of each pathway has demonstrated that their separation is not purely anatomical, but is maintained by molecular interactions with other molecules. Antigen processing has been shown to regulate the overall immune response, but the mechanisms involved remain obscure.

A single amino acid substitution in the Ak molecule fortuitously provokes an alloresponse

We discovered by chance that the R28 T cell hybridoma has dual specificity. It responds to a peptide derived from ribonuclease presented by cells displaying Ak molecules and it reacts, in the absence of added antigen, to cells expressing Ak complexes with a single amino acid substitution at position 69 of the alpha chain. Modelling and functional studies suggest that residue 69 is a peptide contact residue, prompting the hypothesis that R28's alloreactivity is a cross-reactive response to an unknown peptide bound in the 'groove' of the mutant Ak complex. In this report, we employ a competition assay to confirm that this alloresponse involves a groove-binding peptide, demonstrate that this peptide derives from or depends on fetal calf serum and exploit a panel of antigen-presenting cell lines--each displaying an Ak complex with a different position 69 substitution--to establish that the alloresponse is not just a heteroclitic response to ribonuclease, itself. We speculate that much of the alloreactivity against murine class II molecules that is revealed in vitro may prove to be directed at bovine serum-derived peptides, suggesting that in this context, alloreactivity is a misnomer.

T helper cell epitope of rabies virus nucleoprotein defined by tri- and tetrapeptides

T cell clones and subsequently hybridomas were generated from rabies virus-immune C3H/He mice to an immunodominant epitope of the viral nucleoprotein, termed 31D, that had previously been identified by a 15-amino acid-long synthetic peptide. T cells to this epitope that by phenotypical and functional characteristics belonged to the T helper cell subset were shown to respond to most rabies and rabies-related viruses. In order to define the minimal sequence needed to elicit a response from 31D-specific T cell clones or hybridomas, a number of peptides of varied lengths, i.e. 3-32 amino acids long, were tested. The ability of the peptides to induce a response was inversely correlated in their lengths, i.e., short peptides (3-5 amino acids long) had to be used at 10(6) times higher concentrations as compared to long peptides (15 or 32 amino acids long). Conversely, the specificity of the T cell response was directly correlated to the length of the peptides, i.e., while the response to 15-amino acid-long peptides exhibited a high degree of specificity, the response to 3- to 5-amino acid-long peptides showed a high degree of flexibility. The long as well as the short peptides had to be presented in association with I-Ek. We speculate that in this system the T cell receptor interacts predominantly with a peptide-induced modification of the I-Ek molecule.

Chemical and functional analysis of MHC class II-restricted T cell epitopes

Various aspects of antigen degradation and presentation are reviewed, in particular with respect to fragmentation of native vs. denatured proteins, different enzymatic machinery present in different cells and individuals, characterization of epitopes and their persistence on antigen-presenting cells as well as their capacity to interact with different MHC class II molecules. Finally, the structure of antigenic peptides is discussed.

Self recognition by the immune system

In each organism, the immune system must acquire the ability to distinguish self from nonself. Experiments in T cell receptor transgenic mice indicate that this process involves the selection of lymphocytes in the thymus.

Detection of peptide-MHC class II complexes on the surface of intact cells

The interaction of peptides with major histocompatibility complex proteins on the surface of cells is required for their recognition by T lymphocytes. Many studies characterizing the formation of peptide-MHC class II complexes have used either assays for T cell responses or for peptide binding to purified class II molecules. Recently, specific peptide-class II interactions have been demonstrated convincingly on the surface of intact cells. The effects of varying peptide and class II structure have been examined in order to identify structural requirements for binding to cell surface class II molecules and to examine the conformation adopted by immunogenic peptides when bound.

Identification of p70 and p80 associations with class II MHC molecules and Ii

Two proteins, p70 and p80, were found in chemically crosslinked complexes with class II MHC molecules and Ii after 3-12 hr labelings with [35S]methionine. Two-dimensional, nonreduced/reduced SDS gel electrophoresis of immunoprecipitated complexes revealed 1) endogenous disulfide linkages between Ii-Ii and Ii-p70 and 2) chemically crosslinked, nearest neighbors of alpha-beta, alpha-Ii, Ii-p70, and alpha-p80. Although such nearest neighbors within multimeric complexes were identified as dimers in nonreduced/reduced 2D gels, stoichiometries could not be determined in the high molecular weight complex(es), which included alpha, beta, Ii, p70, and p80, and were not separated in the first dimension. p80 was not the chondroitin-sulfate form of Ii (Ii-CS) because it was not electrophoretically heterogeneous and was not sensitive to chondroitinase ABC. p70 was not hsp72/74 detected with C92 or N27 mAbs, and p80 was not BiP detected with its respective mAb. While only these two proteins associated prominently with class II MHC antigens and Ii late after synthesis, their functions are unknown.

Mutations of immunoglobulin transmembrane and cytoplasmic domains: effects on intracellular signaling and antigen presentation

The membrane-bound form of immunoglobulin serves as an antigen-specific receptor for B cells mediating signal transduction and antigen presentation. We have developed an assay that reconstitutes both these physiologic responses with respect to the antigen phosphorylcholine. By introducing specific mutations in the human Ig mu chain gene, we have shown that certain transmembrane residues and the short cytoplasmic domain are crucial for these two activities. Moreover, elimination of a single transmembrane hydroxyl group severely inhibits antigen presentation without affecting signal transduction, suggesting that these two functions are mediated by different protein interactions.

The invariant chain forms complexes with class II major histocompatibility complex molecules and antigenic peptides "in vivo"

The binding of a chicken ovalbumin peptide (residues 323-339), Ova-(323-339), to I-Ad molecules was investigated in vitro and in vivo. By using antigenic peptides labeled either with a hapten or with fluorescein, complexes formed in vitro between I-Ad and antigenic peptides were detected by Western blot analysis with an antibody recognizing the hapten 7-nitrobenzo-2-oxa-1,3-diazole and by scanning gels for fluorescence emitted by fluoresceinated peptide. Both techniques reveal that Ova-(323-339) binds not only to I-Ad alpha/beta heterodimers and separated alpha and beta chains but also to complexes of higher molecular mass. Additional analysis shows that one of these additional complexes contains I-Ad heterodimers, antigenic peptides, and also invariant chain. To explore the physiological role of these complexes, cells were incubated with haptenated peptide and the I-Ad-peptide complexes formed in vivo were purified by affinity chromatography using hapten-specific antibody. The complexes formed migrate with a significantly higher apparent molecular mass than the alpha/beta heterodimers. A band at 180 kDa contained the alpha/beta heterodimer, the antigenic peptide, and the invariant chain. These results show that in vivo high molecular mass complexes formed by the I-Ad heterodimer and the invariant chain bind antigenic peptides.

Evidence of widespread binding of HLA class I molecules to peptides

We have tested the binding of HLA class I proteins to peptides using a solid-phase binding assay. We tested 102 peptides, mostly derived from the HIV gag and HIV pol sequences. Most peptides did not bind to any class I protein tested. The pattern of binding among the three class I proteins tested, HLA-A2, -B27, and -B8, was approximately 85% concordant. Further, all five of the known HIV-1 gag T cell epitopes detected by human CTL bound at least one class I protein. Binding of class I to the peptides could be detected either by directly iodinated class I proteins, or indirectly using monoclonal antibodies specific for class I. The binding to the plates could be blocked with MA2.1, which binds in the alpha 1 region of A2, but not by W6/32, which binds elsewhere. The data presented here show that binding of class I to peptides is specific, but that many peptides bind to more than a single class I protein.

Antigenic and allergenic determinants of ovalbumin--III. MHC Ia-binding peptide (OA 323-339) interacts with human and rabbit specific antibodies

Three analogous peptides mimicking the known primary structure of ovalbumin (OA) in the region 323-339 (namely, OA 323-339, OA 323-338 and OA 324-336) were manually synthesized by the Merrifield solid phase technique. The synthetic preparations were purified by gel filtration and ion exchange high performance liquid chromatographies. The sequence linearities were deduced from the amino acid composition prior to each stage of coupling and the amino acid sequence determination on the ultimate chain. OA 323-339 was conjugated to BSA, using carbodiimide activation of the carrier protein and the conjugated compound was further used for production of antibodies in rabbit. The results showed that the region OA 323-339 was immunogenic; it could give immunoprecipitates with rabbit anti-OA 323-339-BSA in crossed immunoelectrophoresis (CIE). It similarly could bind human specific IgE from serum pools from patients allergic to egg in crossed radio immunoelectrophoresis (CRIE). Quantitative precipitation inhibition and specific IgE inhibition were used for confirming the antigenic and allergenic activities of this region. The results led us to conclude that the region 323-339 of the OA molecule encompassed an allergenic and antigenic epitope which were recognized by human and rabbit B-cells.

Cytotoxic T cell recognition of an endogenous class I HLA peptide presented by a class II HLA molecule

Human leukocytes were stimulated in vitro with peptides corresponding in sequence to the highly variable helix of the alpha 1 domain of various HLA-B and -C molecules. A CD4+ CD8- cytotoxic T cell line, CTL-AV, that is specific for the HLA-B7 peptide presented by HLA-DR11.1 was obtained. The HLA-DR11.2 molecule, which only differs at three residues from HLA-DR11.1, did not present the HLA-B7 peptide to CTL-AV. Peptides from the alpha 1 domain helix of other HLA-A and HLA-B molecules, but not HLA-C molecules, competed with the HLA-B7 peptide for binding to HLA-DR11.1. A cell line (WT50) that coexpresses HLA-B7 and HLA-DR11.1 was killed by CTL-AV in the absence of any added HLA-B7 peptide. The processing and presentation of HLA-B7 in these cells appears to be through the endogenous, and not the exogenous, pathway of antigen presentation. Thus, Brefeldin A inhibits presentation and chloroquine does not. Furthermore, introduction of purified HLA-B7 molecules into HLA-DR11.1+, HLA-B7- cells by cytoplasmic loading via osmotic lysis of pinosomes, but not by simple incubation, rendered them susceptible to CTL-AV killing. These results provide an example of class II major histocompatibility complex (MHC) presentation of a constitutively synthesized self protein that uses the endogenous pathway of antigen presentation. They also emphasize the capacity for presentation of MHC peptides by MHC molecules.

Analysis of physical interactions between peptides and HLA molecules and application to the detection of human immunodeficiency virus 1 antigenic peptides

The physical association of 40 antigenic peptides and purified HLA class I and class II molecules was monitored using a direct peptide binding assay (PBA) in solid phase and an inhibition peptide binding assay (IPBA) in which the competing peptide was present in a soluble phase. We also examined the ability of different peptides to inhibit the lytic activity of human antiviral cytolytic T cells towards cells incubated with the corresponding target peptide. Our results showed that: (a) Binding of a given human T cell-recognized peptide to several HLA class I and class II molecules occurred frequently. Nevertheless, preferential binding of peptides to their respective restriction molecules was also observed. (b) Binding of HLA molecules to peptides recognized by murine T cells occurred less frequently. (c) 11 of 24 (46%) randomly selected HIV-1 peptides contained agretopic residues allowing their binding to HLA molecules. (d) The kinetics of HLA/peptide association depended on the peptide tested and were faster than or similar to those reported for Ia molecules. Dissociation of these complexes was very low. (e) Peptide/HLA molecule binding was dependent on length, number of positive charges, and presence of hydrophobic residue in the peptide. (f) A correlation was demonstrated between a peptide inhibitory effect in the IPBA and its blocking effect in the cytolytic test. Our data indicated that the restriction phenomenon observed in T cell responses was not strictly related to either an elective HLA/peptide association, or a high binding capacity of a peptide to HLA molecules. These data also showed that the PBA and IPBA are appropriate for the detection of agretopic residues within HIV-1 proteins.

Dendritic cells pulsed with protein antigens in vitro can prime antigen-specific, MHC-restricted T cells in situ

T cells recognize peptides that are bound to MHC molecules on the surface of different types of antigen-presenting cells (APC). Antigen presentation most often is studied using T cells that have undergone priming in situ, or cell lines that have been chronically stimulated in vitro. The use of primed cells provides sufficient numbers of antigen-reactive lymphocytes for experimental study. A more complete understanding of immunogenicity, however, requires that one develop systems for studying the onset of a T cell response from unprimed lymphocytes, especially in situ. Here it is shown that mouse T cells can be reliably primed in situ using dendritic cells as APC. The dendritic cells were isolated from spleen, pulsed with protein antigens, and then administered to naive mice. Antigen-responsive T cells developed in the draining lymphoid tissue, and these T cells only recognized protein when presented on cells bearing the same MHC products as the original priming dendritic cells. In contrast, little or no priming was seen if antigen-pulsed spleen cells or peritoneal cells were injected. Since very small amounts of the foreign protein were visualized within endocytic vacuoles of antigen-pulsed dendritic cells, it is suggested that dendritic cells have a small but relevant vacuolar system for presenting antigens over a several day period in situ.

Malaria antigens and MHC restriction

In the case of the malaria CS protein we have shown that there is at least one T cell determinant which is able to bind to and be recognized by most human MHC class II molecules, while for the 190L polypeptide, derived from a conserved region of the p190 merozoite surface protein, we have identified several epitopes recognized by T cell clones in association with different HLA-class II isotypes and alleles. In addition, binding analysis of these epitopes indicated that most of the peptides are able to bind to multiple allelic forms of class II molecules. Although there are important obstacles to malaria vaccine development we believe that, in the light of these results, unresponsiveness in humans, caused by MHC restriction, might not be a major constraint in development of a subunit vaccine.

Human homologue of murine tumor rejection antigen gp96: 5'-regulatory and coding regions and relationship to stress-induced proteins

Cell-surface glycoproteins of 96 kDa (gp96) have been implicated in immunogenicity of methylcholanthrene-induced mouse sarcomas in syngeneic hosts. In view of the potential immunogenicity of gp96-related molecules in human tumors, we have defined the 5'-regulatory and complete coding regions of a human gp96 transcript. The 5'-regulatory region contains an imperfect heat shock element apart from other regulatory sequences. The amino acid sequence of human gp96 is 96% homologous to its murine counterpart and genes for the two molecules show significant homology between untranslated regions. Comparison of gp96 sequences to other sequences in DNA and protein data bases indicates significant homology with the stress proteins 94-kDa glucose-regulated protein (grp94) and 108-kDa heat shock protein (hsp108) and 99-kDa endoplasmic reticular protein (ERp99). These molecules are either identical or represent a family of closely related molecules. With regard to their role in tumor immunity, it needs to be determined whether gp96 molecules are tumor antigens per se or whether they serve as carriers of other immunogenic moieties.

The role of polymorphic HLA-DR beta chain residues in presentation of viral antigens to T cells

The relative importance of 11 polymorphic positions in the HLA-DR7 beta 1 chain in T cell recognition of foreign antigens was investigated using transfectants expressing mutant DR7 beta 1 chains as APC for five rabies virus-specific T cell clones. The results indicate that multiple amino acids, located in both the beta-strands and alpha-helix of DR7 beta 1 in the model of a class II molecule, are involved in DR7-restricted T cell recognition of these antigens. Many of the substitutions appeared to reduce the affinity of an antigenic peptide for the mutant DR7 molecules but did not prevent binding. The heterogeneity of responses of the three G-specific T cell clones to presentation of the G11.3 peptide by several of the mutant DR7 molecules indicates that the T cell receptor (TCR) of each these clones requires a different view of the G11.3/DR7 complex and raises the possibility that the G11.3 peptide may bind to the DR7 molecule in more than one conformation.

The role of the T cell receptor in positive and negative selection of developing T cells

Although many combinations of alpha beta T cell receptors are available to the T cells in any given organism, far fewer are actually used by mature T cells. The combinations used are limited by two selective processes, positive selection of T cells bearing receptors that will be useful to the host, and clonal elimination or inactivation of T cells bearing receptors that will be damaging to the host. The ways in which these two apparently contradictory processes occur, and the hypotheses that have been suggested to reconcile them, are discussed.

Specific killing of cytotoxic T cells and antigen-presenting cells by CD4+ cytotoxic T cell clones. A novel potentially immunoregulatory T-T cell interaction in man

Mycobacterial antigens not only stimulate Th cells that produce macrophage-activating factors, but also CD4+ and CD8+ CTL that lyse human macrophages. The mycobacterial recombinant 65-kD hsp was previously found to be an important target antigen for polyclonal CD4+ CTL. Because of the major role of 65-kD hsp in the immune response to mycobacterial as well as autoantigens, we have studied CTL activity to this protein at the clonal level. HLA-DR or HLA-DQ restricted, CD4+CD8- T cell clones that recognize different peptides of the M. leprae 65-kD hsp strongly lysed EBV-BLCL pulsed with specific but not irrelevant peptide. No bystander lysis of B cells, T cells, or tumor cells was seen. Target cell lysis could not be triggered by PMA + Ca2+ ionophore alone and depended on active metabolism. Interestingly, these CD4+ CTL also strongly lysed themselves and other HLA-class II compatible CD4+ (TCR-alpha/beta or -gamma/delta) or CD8+ CTL clones in the presence of peptide, suggesting that CTL are not actively protected from CTL-mediated lysis. Cold target competition experiments suggested that EBV-BLCL targets were more efficiently recognized than CD4+ CTL targets. These results demonstrate that hsp65 peptide-specific HLA class II-restricted CD4+ T cell clones display strong peptide-dependent cytolytic activity towards both APCs, and, unexpectedly, CD4+ and CD8+ CTL clones, including themselves. Since, in contrast to murine T cells human T cells express class II, CTL-mediated T cell killing may represent a novel immunoregulatory pathway in man.

Antigen presentation and self-nonself discrimination

Self-nonself discrimination is primarily carried out by T cells. Since the ligand recognized by T cells is a complex formed by antigenic peptides bound to MHC molecules, positive and negative selection of T lymphocytes must be based on the recognition of complexes formed by self-peptides bound to MHC molecules. This requires that self-antigens are continuously processed, bound by MHC molecules, and presented to T cells under conditions inducing both positive selection of T cells potentially able to recognize foreign antigens and negative selection, either by physical deletion or functional inactivation, of potentially autoreactive T cells. Self-nonself discrimination is not confined to intrathymic development of T lymphocytes, but it is a continuing process among peripheral T cells. Accordingly, autoimmunity is induced when self-antigens, or foreign antigens cross-reactive with self antigens, bound to MHC molecules, are presented under conditions able to activate self-reactive T cells. Based on these premises, a way of interfering with the induction of autoimmune diseases could rely on blocking the MHC binding site presenting the autoantigen.

Peptide binding to HLA-DR1: a peptide with most residues substituted to alanine retains MHC binding

Major histocompatibility complex (MHC) glycoproteins play an important role in the development of an effective immune response. An important MHC function is the ability to bind and present 'processed antigens' (peptides) to T cells. We show here that the purified human class II MHC molecule, HLA-DR1, binds peptides that have been shown to be immunogenic in vivo. Detergent-solubilized HLA-DR1 and a papain-cleaved form of the protein lacking the transmembrane and intracellular regions have similar peptide binding properties. A total of 39 single substitutions were made throughout an HLA-DR1 restricted hemagglutinin epitope and the results determine one amino acid in this peptide which is crucial to binding. Based on this analysis, a synthetic peptide was designed containing two residues from the original hemagglutinin epitope embedded in a chain of polyalanine. This peptide binds to HLA-DR1, indicating that the majority of peptide side chains are not required for high affinity peptide binding.

Invariant chain influences the immunological recognition of MHC class II molecules

Recent experiments have implicated intracellular events in the formation of the MHC class II-peptide complexes recognized by CD4-positive T cells. These data raise the possibility that the intracellular association of class II with the non-polymorphic glycoprotein, invariant chain (Ii), may regulate the interaction between processed antigen and MHC class II molecules. To address this possibility, we have generated a series of transfected fibroblast cell lines that express class II with and without Ii. Although the presence of Ii does not seem to affect the ability of the cells to process and present intact antigen, Ii-negative cells express an altered form of class II at the cell surface. This modified conformation of class II in Ii-negative cells is detectable by an increase in the ability to present antigenic peptides to T cells and a decrease in the binding of several class II-specific monoclonal antibodies.

MHC class II-derived peptides can bind to class II molecules, including self molecules, and prevent antigen presentation

Seven synthetic peptides corresponding to the polymorphic regions of the alpha and beta chains of the I-Ak molecule were examined for their ability to inhibit the presentation of foreign antigens to antigen-specific, I-A-restricted T cell hybridomas. Two of the peptides, representing the sequences found in the first and third polymorphic regions (PMR) of the A alpha k chain (alpha k-1 and alpha k-3) were capable of inhibiting the presentation of three different HEL-derived peptide antigens to their appropriate T cells. In addition, the alpha k-1 peptide inhibited the presentation of the OVA(323-339) immunodominant peptide to the I-Ad-restricted T cell hybridomas specific for it. Prepulsing experiments demonstrated that the PMR peptides were interacting with the APC and not with the T cell hybridomas. These observations were confirmed and extended by the demonstration that the alpha k-1 and alpha k-3 peptides blocked the direct binding of HEL(46-61) to purified I-Ak and that the alpha k-1 peptide blocked the binding of OVA(323-339) to I-Ad. The binding competition experiments suggest that the alpha k-1 peptide binds to the I-Ak molecule from which it was derived with a Kd approximately 10(-5) M, while the alpha k-3 peptide binds slightly less well. These combined data, suggesting that class II-derived peptides can bind to MHC class II molecules, including the autologous molecule from which they are derived, have important implications for the molecular basis of alloreactivity and autoreactivity. Further, they suggest a possible mechanism by which selecting elements, involving only MHC molecules, may be generated in the thymus.