The relationship between class I binding affinity and immunogenicity of potential cytotoxic T cell epitopes

The relationship between binding affinity for HLA class I molecules and immunogenicity of discrete peptide epitopes has been analyzed in two different experimental approaches. In the first approach, the immunogenicity of potential epitopes ranging in MHC binding affinity over a 10,000-fold range was analyzed in HLA-A*0201 transgenic mice. In the second approach, the antigenicity of approximately 100 different hepatitis B virus (HBV)-derived potential epitopes, all carrying A*0201 binding motifs, was assessed by using PBL of acute hepatitis patients. In both cases, it was found that an affinity threshold of approximately 500 nM (preferably 50 nM or less) apparently determines the capacity of a peptide epitope to elicit a CTL response. These data correlate well with class I binding affinity measurements of either naturally processed peptides or previously described T cell epitopes. Taken together, these data have important implications for the selection of epitopes for peptide-based vaccines, and also formally demonstrate the crucial role of determinant selection in the shaping of T cell responses. Because in most (but not all) cases, high affinity peptides seem to be immunogenic, our data also suggest that holes in the functional T cell repertoire, if they exist, may be relatively rare.

Peptide binding to the most frequent HLA-A class I alleles measured by quantitative molecular binding assays

Quantitative assays to measure the binding of defined synthetic antigenic peptides and purified MHC class I molecules are described for several common human HLA-A alleles (A1, A2.1, A3, A11 and A24). Under appropriate conditions, the binding of radiolabeled peptides to purified MHC class I molecules is very effective, highly specific, and appears to be dependent on the specific sequence motif of the peptide as defined by critical anchor residue positions. Establishment and optimization of the assay reveals that a relatively high fraction of the MHC class I molecules isolated from EBV transformed B cell line sources is capable of binding exogenously added peptide. Scatchard analysis for all alleles yields 5-10% occupancy values. There is a stringent peptide size requirement that is reflected by the direct influence of peptide length on the binding affinity. The peptide-MHC class I interactions demonstrate remarkable similarity to peptide-MHC class II interactions, both in overall affinity and kinetic behavior. The immunological relevance of the peptide-MHC class I binding assay is also demonstrated by measuring the affinity of a panel of previously described HLA restricted peptides for their HLA restriction element. In 91% (10/11) of the cases, the peptides bound with affinities of 50 nM or less, and in the remaining 9% (1/11) of the cases, in the 50 to 500 nM range. Thus, these data provide the first quantitative estimate of what level of HLA-A binding affinity is associated with a diverse panel of immunodominant CTL epitopes in man.

Class I MHC-peptide interaction: structural and functional aspects

The structural requirements for the interaction between antigens and class I molecules was investigated through the use of a quantitative assay to measure peptide binding to different MHC class I alleles. We determined the permissiveness of the main anchors reported by Rammensee and his group for peptide binding and defined an extended motif for peptides binding to the HLA-A2.1 allele, including the role of non-anchor positions. It was found that the main anchors were necessary, but not sufficient, for good binding. Certain non-anchor positions contributed significantly to overall binding and were referred to a secondary anchors. This finding allowed a better prediction of high affinity binding peptides selected from libraries of different viral and tumor proteins. Furthermore, our data allowed correlation of the structural requirements for binding of peptides with crystallographic data of the MHC molecule. In order to characterize allele-specific motifs for a larger number of alleles, the HLA-A alleles A1, A3, A11, and A24, which represent some of the most common alleles found in different ethnic populations, were chosen. Here, most motifs were found to be highly exclusive; however, HLA-A3 and A11 shared a common motif. The defined motifs were validated further by using naturally processed peptides. Those peptides were also synthesized and tested for binding to the appropriate HLA alleles, giving a binding affinity from 0.3 to 200 nM for sequences of naturally processed peptides. Finally, a set of all possible 9-mer peptides from HPV 16 proteins were synthesized and tested for binding to the five class I alleles. For each allele, high affinity binders were identified, thus allowing for selection of possible peptide candidates for a CTL based vaccine.

Failure to demonstrate long-lived MHC saturation both in vitro and in vivo. Implications for therapeutic potential of MHC-blocking peptides

Peptides that bind with high affinity to class II MHC molecules can inhibit T cell activation both in vitro and in vivo. Thus, they have been suggested as potential therapeutic agents for MHC-associated autoimmune diseases. We have constructed nonnatural peptides with high affinity for certain disease-associated MHC alleles. More specifically, a particular peptide, designated as CY-760.50, was found to have a high binding affinity for DR1, slow dissociation kinetics after binding to MHC, and prolonged stability in human serum. However, when the ability of this peptide to block peptide presentation to an influenza hemagglutinin 307-319 peptide-specific, DR1-restricted T cell clone was examined, it was found that MHC blockade could only be achieved when high concentrations of peptide were present along with Ag in the fluid phase. Thus, pretreatment of APC with MHC class II blocker, followed by removal of unbound blocker, did not result in saturation of MHC molecules, because practically immediate reacquisition of Ag-presenting capacity was observed after removal of fluid phase blocker. The pharmacokinetic behavior and the duration of blocking activity of CY-760.50 were also examined in vivo, taking advantage of the fact that the mouse MHC class II molecule I-Ab also bound CY-760.50 with high affinity. CY-760.50 administered i.v. to C57BL/6 mice was rapidly cleared from the circulation and virtually undetectable in the serum 10 min after injection. This fast clearance rate was paralleled by a similarly short duration of the MHC blockade effect. These in vivo results have implications concerning the biology of peptide-MHC interactions, and suggest that MHC blockade may not be feasible as a therapeutic approach unless effective concentrations of inhibitor can be maintained over extended periods of time in the extracellular fluids.

Failure to demonstrate long-lived MHC saturation both in vitro and in vivo. Implications for therapeutic potential of MHC-blocking peptides

Peptides that bind with high affinity to class II MHC molecules can inhibit T cell activation both in vitro and in vivo. Thus, they have been suggested as potential therapeutic agents for MHC-associated autoimmune diseases. We have constructed nonnatural peptides with high affinity for certain disease-associated MHC alleles. More specifically, a particular peptide, designated as CY-760.50, was found to have a high binding affinity for DR1, slow dissociation kinetics after binding to MHC, and prolonged stability in human serum. However, when the ability of this peptide to block peptide presentation to an influenza hemagglutinin 307-319 peptide-specific, DR1-restricted T cell clone was examined, it was found that MHC blockade could only be achieved when high concentrations of peptide were present along with Ag in the fluid phase. Thus, pretreatment of APC with MHC class II blocker, followed by removal of unbound blocker, did not result in saturation of MHC molecules, because practically immediate reacquisition of Ag-presenting capacity was observed after removal of fluid phase blocker. The pharmacokinetic behavior and the duration of blocking activity of CY-760.50 were also examined in vivo, taking advantage of the fact that the mouse MHC class II molecule I-Ab also bound CY-760.50 with high affinity. CY-760.50 administered i.v. to C57BL/6 mice was rapidly cleared from the circulation and virtually undetectable in the serum 10 min after injection. This fast clearance rate was paralleled by a similarly short duration of the MHC blockade effect. These in vivo results have implications concerning the biology of peptide-MHC interactions, and suggest that MHC blockade may not be feasible as a therapeutic approach unless effective concentrations of inhibitor can be maintained over extended periods of time in the extracellular fluids.

Negative selection of CD4+ CD8+ thymocytes by T-cell receptor peptide antagonists

Antigen-induced activation of T cells can be specifically inhibited by antigen analogs that have been termed T-cell receptor peptide antagonists. These antagonists appear to act by inducing the formation of nonstimulatory or partially stimulatory complexes between T-cell receptors and the major histocompatibility complex molecules presenting the peptides. Herein, we have investigated the effect of T-cell receptor peptide antagonists on thymocyte negative selection. First, peptide antagonists were identified for the cytochrome c-specific T-cell clone AD10. These peptides were then tested for their ability to induce negative selection in an in vitro model system using thymocytes from mice transgenic for the AD10 T-cell receptor. Though unable to induce mature T-cell activation, the T-cell receptor peptide antagonists induced deletion of CD4+ CD8+ thymocytes. These results suggest that negative selection of CD4+ CD8+ thymocytes can be induced by T-cell receptor interactions of a lower affinity than those required for mature T-cell activation.

Definition of specific peptide motifs for four major HLA-A alleles

Allele-specific motifs for the human MHC class I molecules, HLA-A1, A3, A11, and A24 were characterized by three complementary approaches. First, amino acid sequence analysis of acid eluted peptide pools from affinity purified class I molecules defined putative motifs 9 or 10 amino acids in length and bearing critical anchor residues at position 2 and at the COOH-terminal. These motifs were distinct, with the exception of the HLA-A3 and A11 motifs that were very similar to each other. Second, the correctness of these putative motifs was verified by analyzing the binding capacity of polyalanine peptide analogues to purified HLA-A molecules. Several alternative anchor residues that were not obvious from the pooled peptide sequencing analysis were identified. Third, sequences of individual peptides eluted from HLA-A1, A11, and A24 were determined by tandem mass spectrometry. Nonamers were the predominant species, although peptides of 8, 10, 11, and 12 amino acids in length were also identified. These peptides displayed anchor residues predicted by the specific motifs at position 2 and at the COOH-terminal, regardless of peptide length. Synthetic versions of the naturally processed peptides were shown to bind to the appropriate HLA-A alleles with IC50 values in the 0.3- to 200-nM range. A rational approach to search Ags with known amino acid sequences for epitopes restricted by some of the most common HLA-A types and of potential clinical importance is now feasible.

Role of HLA-A motifs in identification of potential CTL epitopes in human papillomavirus type 16 E6 and E7 proteins

We have measured the binding affinity for five HLA-A alleles: HLA-A1 (A*0101), A2.1 (A*0201), A3 (A*0301), A11 (A*1101), and A24 (A*2401); of a set of all possible nonamer peptides (n = 240) of human papillomavirus type 16 E6 and E7 proteins. High affinity binding peptides were identified for each of the alleles, thus allowing us to select several candidates for CTL-based vaccines. Moreover, this unbiased set of peptides allowed an evaluation of the predictive value of HLA motifs derived either from the analysis of sequencing of pools of naturally processed peptides or from the binding analysis of polyalanine nonameric peptides that differed in the amino acids (aa) present at the anchor positions. Whereas pool sequencing-derived motifs were present in only 27% of high affinity binders, the more expanded motif, based on analysis of different aa substitutions at the anchor positions, was present in 73% of high affinity binders. Furthermore, it was found that the presence of anchor residues in a peptide was in itself not sufficient to determine binding to MHC class I molecules, because the majority of motif-containing peptides failed to bind to the relevant MHC. Finally, specific HLA motifs were used to predict peptide binders of 8, 10, and 11 aa in length. Several high affinity binding peptides were identified for each of the various peptide lengths, indicating a significant size heterogeneity in peptides capable of high affinity binding to HLA-A molecules.

Definition of specific peptide motifs for four major HLA-A alleles

Allele-specific motifs for the human MHC class I molecules, HLA-A1, A3, A11, and A24 were characterized by three complementary approaches. First, amino acid sequence analysis of acid eluted peptide pools from affinity purified class I molecules defined putative motifs 9 or 10 amino acids in length and bearing critical anchor residues at position 2 and at the COOH-terminal. These motifs were distinct, with the exception of the HLA-A3 and A11 motifs that were very similar to each other. Second, the correctness of these putative motifs was verified by analyzing the binding capacity of polyalanine peptide analogues to purified HLA-A molecules. Several alternative anchor residues that were not obvious from the pooled peptide sequencing analysis were identified. Third, sequences of individual peptides eluted from HLA-A1, A11, and A24 were determined by tandem mass spectrometry. Nonamers were the predominant species, although peptides of 8, 10, 11, and 12 amino acids in length were also identified. These peptides displayed anchor residues predicted by the specific motifs at position 2 and at the COOH-terminal, regardless of peptide length. Synthetic versions of the naturally processed peptides were shown to bind to the appropriate HLA-A alleles with IC50 values in the 0.3- to 200-nM range. A rational approach to search Ags with known amino acid sequences for epitopes restricted by some of the most common HLA-A types and of potential clinical importance is now feasible.

Role of HLA-A motifs in identification of potential CTL epitopes in human papillomavirus type 16 E6 and E7 proteins

We have measured the binding affinity for five HLA-A alleles: HLA-A1 (A*0101), A2.1 (A*0201), A3 (A*0301), A11 (A*1101), and A24 (A*2401); of a set of all possible nonamer peptides (n = 240) of human papillomavirus type 16 E6 and E7 proteins. High affinity binding peptides were identified for each of the alleles, thus allowing us to select several candidates for CTL-based vaccines. Moreover, this unbiased set of peptides allowed an evaluation of the predictive value of HLA motifs derived either from the analysis of sequencing of pools of naturally processed peptides or from the binding analysis of polyalanine nonameric peptides that differed in the amino acids (aa) present at the anchor positions. Whereas pool sequencing-derived motifs were present in only 27% of high affinity binders, the more expanded motif, based on analysis of different aa substitutions at the anchor positions, was present in 73% of high affinity binders. Furthermore, it was found that the presence of anchor residues in a peptide was in itself not sufficient to determine binding to MHC class I molecules, because the majority of motif-containing peptides failed to bind to the relevant MHC. Finally, specific HLA motifs were used to predict peptide binders of 8, 10, and 11 aa in length. Several high affinity binding peptides were identified for each of the various peptide lengths, indicating a significant size heterogeneity in peptides capable of high affinity binding to HLA-A molecules.

T cell receptor antagonist peptides are highly effective inhibitors of experimental allergic encephalomyelitis

The feasibility of using T cell receptor (TcR) antagonist peptides to inhibit autoimmune disease has been examined. First, the fine antigenic structure of the I-As-restricted encephalitogenic determinant proteolipid protein (PLP) 139-151 has been analyzed. It was found that residues 145 and 148 were I-As anchor residues, and residue 144 appeared to be especially critical in T cell activation. Residues 142, 143, 146, and 147 were found to be crucial for activation of some, but not all, of the T cells studied. Next, good I-As-binding nonantigenic analogs were tested for TcR antagonism. Accordingly, several single substitution analogs were identified which could act as TcR antagonists. Moreover, when two such analogs were combined, the resulting TcR antagonist pool inhibited most of the PLP 139-151-specific T cell clones in vitro. When the efficacy of this TcR antagonist pool in inhibiting EAE induction in vivo was examined, it was found that the analog pool was a remarkably potent inhibitor of disease induction. The TcR antagonist pool was approximately 10-fold more potent than our best major histocompatibility complex blocker and was still capable of significant inhibition when injected in equimolar amounts with the encephalitogenic PLP 139-151 determinant.

Antigen analogs/MHC complexes as specific T cell receptor antagonists

Recent studies demonstrated that antigen analogs can act as powerful and specific inhibitors of T cell activation, leading to the formulation of the concept that antigen analog/MHC complexes may act as antagonists of the T cell receptor (TCR). TCR antagonism appears to be associated with engagement of the TCR below a crucial affinity threshold necessary for full T cell activation. Studies addressing the molecular mechanism of this effect suggest that TCR antagonists could act by interfering with membrane-related events (such as proper receptor clustering) that might precede intracellular signaling. Discovery of the TCR antagonism phenomenon also suggested a possible rational approach to antigen-specific immunointervention in allergies and autoimmune diseases. The feasibility of such an approach is now being actively investigated. Finally, TCR antagonist peptides may provide a useful tool to probe TCR-peptide/MHC interactions involved in the process of thymic education.

Peripheral T cell response to A-gliadin in celiac disease: differential processing and presentation capacities of Epstein-Barr-transformed B cells and fibroblasts

Celiac disease (CD) is a small intestinal disorder characterized by the malabsorption of most nutrients. Disease pathogenesis appears to be associated with immune-mediated pathology. Susceptibility is associated with genes coding for DQw2 class II molecules. In the present report we investigated T cell responses to A-gliadin (AGL), a major alpha-gliadin component known to activate disease. Gliadin-specific lines were generated from a CD patient and a normal donor. Three major points were revealed by the analysis of these T cells: (1) On the basis of mapping experiments using Epstein-Barr virus (EBV) lines and DR-transfected fibroblasts and DR-, DP-, and DQ-specific monoclonal antibodies (mAb), all responses appeared to be DR-restricted. Thus, in contrast to the strong association of disease susceptibility with DQ molecules, no DQ-restricted, gliadin-specific response was detectable. (2) Fine specificity analysis, using a panel of synthetic peptides spanning the entire alpha-gliadin component molecule, revealed that the clones derived from the normal donor were DR53-restricted and AGL 21-40-specific, while clones derived from the CD patient were DR7-restricted and peptide 1-20-specific. (3) Both whole AGL and AGL 1-20 were presented to the patient-derived clones with much higher efficiency by DDR-transfected fibroblasts than by EBV lines. These data suggested that fibroblasts processed this determinant efficiently, while EBV lines were unable to do so. Indeed, analysis of a panel of truncated AGL 1-20 analogs revealed that peptide AGL 1-8, which contained the minimal T cell epitope, was presented with equal efficiency by fixed or irradiated EBV and irradiated DR7-transfected fibroblasts.

Induction of anti-tumor cytotoxic T lymphocytes in normal humans using primary cultures and synthetic peptide epitopes

Cytotoxic T lymphocytes (CTLs) recognize peptide antigens associated with cell surface major histocompatibility complex (MHC) molecules. The identification of tumor cell-derived peptides capable of eliciting anti-tumor CTL responses would enable the design of antigen-specific immunotherapies. Our strategy to identify such potentially therapeutic peptides relies on selecting high-affinity MHC binders from known tumor-associated antigens. These peptides are subsequently tested for their ability to induce CTLs capable of killing tumor cells. With this strategy, we have identified a nine-residue epitope, derived from the product of the tumor-associated gene MAGE-3, which has the capacity to induce in vitro CTLs that kill melanoma and other tumor cell lines. These results show the primary in vitro induction of tumor-specific human CTLs and illustrate the feasibility of ex vivo antigen-specific approaches to the immunological therapy of cancer.

The inhibition of different T cell lines specific for the same antigen with TCR antagonist peptides

To further understand and evaluate the phenomenon of TCR antagonism, we wished to determine whether analogs of an antigenic determinant could antagonize a specific polyclonal response. To this end, the ability of TCR antagonist peptides to inhibit a panel of five different DR4w4-restricted, influenza hemagglutinin 307-319-specific T cell lines was examined. An analysis of their V beta and J beta usage indicated that each of these five T cell lines expressed different TCR. A series of HA 307-319 single amino acid substituted analogs were used to determine the fine Ag specificities of the different lines. Ag analogs that demonstrated little or no stimulatory capacity were then examined for their ability to act as TCR antagonists by inhibiting the proliferative response of these five lines. Overall, 17 different peptide analogs capable of antagonizing at least one T cell line were identified. Although no single analog was capable of inhibiting all five T cell lines, two different analogs were identified that were capable of inhibiting four of five of the T cell specificities examined.

The inhibition of different T cell lines specific for the same antigen with TCR antagonist peptides

To further understand and evaluate the phenomenon of TCR antagonism, we wished to determine whether analogs of an antigenic determinant could antagonize a specific polyclonal response. To this end, the ability of TCR antagonist peptides to inhibit a panel of five different DR4w4-restricted, influenza hemagglutinin 307-319-specific T cell lines was examined. An analysis of their V beta and J beta usage indicated that each of these five T cell lines expressed different TCR. A series of HA 307-319 single amino acid substituted analogs were used to determine the fine Ag specificities of the different lines. Ag analogs that demonstrated little or no stimulatory capacity were then examined for their ability to act as TCR antagonists by inhibiting the proliferative response of these five lines. Overall, 17 different peptide analogs capable of antagonizing at least one T cell line were identified. Although no single analog was capable of inhibiting all five T cell lines, two different analogs were identified that were capable of inhibiting four of five of the T cell specificities examined.

HLA DR4w4-binding motifs illustrate the biochemical basis of degeneracy and specificity in peptide-DR interactions

In the present study, we describe the definition of a DRB1*0401 (DR4w4)-specific motif. The strategy used entailed a three-step process: 1) screening a large set of unrelated peptide ligands to identify good MHC binders; 2) truncation analysis of several DR4w4 binding peptides of high affinity to identify the crucial core-binding regions; 3) the use of single amino acid substitutions of the DR4w4-binding peptide hemagglutinin (HA) 307-319 to elucidate the specific residues crucial for binding. The DR4w4 motif is characterized by the presence of a hydrophobic or aromatic (F, W, Y, L, I, V, M) anchor residue in position 1, and a second hydroxyl (S, T) or aliphatic (L, I, V, or M) anchor residue in position 6. Furthermore, positive charges (R, K) are not allowed in positions 4, 7, and 9, and negative charges (D, E) are not allowed in position 9. This motif was present in 92% of good (IC50 < or = 100 nM) DR4w4-binding peptides, but less than 25% of the negative (IC50 > 45 microM) binders, indicating that the presence of the motif is necessary, but not sufficient for good DR4w4 binding capacity. The results of the present study are discussed in relation to previous work defining binding motifs and rules for other DR alleles, illustrating how different DR alleles bind variations on a similar structural theme. Finally, using two different peptide ligands [tetanus toxoid 830-843 and HA 307-319] as model systems, it is demonstrated how the fine allelic specificity of the DR binders can be predictably modulated by introducing subtle changes in the primary amino acid sequence.

HLA DR4w4-binding motifs illustrate the biochemical basis of degeneracy and specificity in peptide-DR interactions

In the present study, we describe the definition of a DRB1*0401 (DR4w4)-specific motif. The strategy used entailed a three-step process: 1) screening a large set of unrelated peptide ligands to identify good MHC binders; 2) truncation analysis of several DR4w4 binding peptides of high affinity to identify the crucial core-binding regions; 3) the use of single amino acid substitutions of the DR4w4-binding peptide hemagglutinin (HA) 307-319 to elucidate the specific residues crucial for binding. The DR4w4 motif is characterized by the presence of a hydrophobic or aromatic (F, W, Y, L, I, V, M) anchor residue in position 1, and a second hydroxyl (S, T) or aliphatic (L, I, V, or M) anchor residue in position 6. Furthermore, positive charges (R, K) are not allowed in positions 4, 7, and 9, and negative charges (D, E) are not allowed in position 9. This motif was present in 92% of good (IC50 < or = 100 nM) DR4w4-binding peptides, but less than 25% of the negative (IC50 > 45 microM) binders, indicating that the presence of the motif is necessary, but not sufficient for good DR4w4 binding capacity. The results of the present study are discussed in relation to previous work defining binding motifs and rules for other DR alleles, illustrating how different DR alleles bind variations on a similar structural theme. Finally, using two different peptide ligands [tetanus toxoid 830-843 and HA 307-319] as model systems, it is demonstrated how the fine allelic specificity of the DR binders can be predictably modulated by introducing subtle changes in the primary amino acid sequence.

Prominent role of secondary anchor residues in peptide binding to HLA-A2.1 molecules

The functional determinants of histocompatibility leukocyte antigen (HLA)-A2.1-peptide interactions have been detailed by the use of quantitative molecular binding assays and a chemically synthesized library of naturally occurring epitopes. The importance of hydrophobic anchor residues in position 2 and the C-terminus was confirmed. These anchors are necessary, but not sufficient, for high affinity binding, as the predictions based solely on these anchors are only about 30% accurate. Prominent roles for several other positions (1, 3, and 7) were also demonstrated. The location of these residues within the peptides matches secondary A2.1 pockets previously demonstrated by X-ray crystallography. From a functional standpoint, similar dominant negative effects on binding were observed for charged residues in both nonamers and decamers, while positive effects differed between nonamers and decamers. An extended motif taking into account secondary anchors increased the predictability of A2.1-binding epitopes to a level of 70%, underscoring the practical usefulness of extended motifs.

Effect of T-cell receptor antagonism on interaction between T cells and antigen-presenting cells and on T-cell signaling events

T-cell receptor (TCR) antagonism induced by complexes of antigen analogue with major histocompatibility complex (MHC) molecules results in efficient inhibition of antigen-dependent T-cell responses. We have investigated some of the possible mechanisms by which TCR antagonists bound to the MHC molecules of antigen-presenting cells (APCs) can inhibit T-cell activation. Using a nonstimulatory analogue of the antigenic peptide influenza hemagglutinin-(307-319), we showed that MHC/antagonist complexes completely inhibit very early intracellular events of antigen-dependent T-cell activation, such as inositol phosphate turnover and Ca2+ influx. In a parallel series of experiments, the effect of TCR antagonist peptide on membrane-related activation events was also investigated. It was found that MHC/antagonist complexes on the surface of APCs did not induce stable conjugates with T cells and, most interestingly, did not inhibit antigen-induced conjugate formation. Thus, our data suggest that antagonistic peptides do not interfere with the cellular events that are required for stable T-cell/APC conjugate formation but do inhibit early biochemical events required for T-cell proliferation. The data are discussed with respect to the role of surface receptor clustering in TCR antagonism.

Antigen analogues as antagonists of the T cell receptor

Complexes of antigen analogues and major histocompatibility complexes have been demonstrated to function as effective antagonists of the T cell receptor (TCR). It was observed that modification of any of the major T cell contact residues can create powerful TCR antagonists. Increasing similarity of antagonist to antigen structure resulted in increased capacity to act as a TCR antagonist up to a point beyond which the analogues themselves showed antigenicity. These data strongly suggested that peptide: TCR interaction with a certain low affinity may still be sufficient for engagement of the receptor but not for signalling, thus resulting in antagonism. It was found that the presentation of antagonistic peptides alone did not induce the formation of stable conjugates between antigen presenting cells and T cells, but rather that presentation of antigen was required to induce the initial interaction of APC with T cells in cell:cell conjugates. This antigen-dependent conjugate formation was not affected by the antagonist, while very early intracellular biochemical events such as PI turnover and CA2+ flux were inhibited.

Peptides presented to the immune system by the murine class II major histocompatibility complex molecule I-Ad

Between 650 and 2000 different peptides are associated with the major histocompatibility complex class II molecule I-Ad. Sequences for nine of these were obtained by a combination of automated Edman degradation and tandem mass spectrometry. All of the peptides are derived from secretory or integral membrane proteins that are synthesized by the antigen-presenting cell itself. Peptides were 16 to 18 residues long, had ragged NH2-and COOH-termini, and contained a six-residue binding motif that was variably placed within the peptide chain. Binding data on truncated peptides suggest that the peptide binding groove on class II molecules can be open at both ends.