A novel approach to the generation of high affinity class II-binding peptides

We have found that if core regions crucial for class II binding are incorporated in multiple copies in the same peptide molecule ("reiterative motifs"), marked enhancement of the binding capacity occurs. Isotype specificity (IAd vs IEd binding capacities) is retained in all three antigenic determinants so far analyzed (lambda rep 12-26, OVA 323-339, and hen egg lysozyme 105-120). The mechanism involved in such an effect is not clear, but experiments involving introduction of a peptide spacer between two repeated core regions do not support the notion that the effect is mediated by cross-linking of more than one MHC molecule, favoring the possibility that conformational effects or distinct subsites of interaction on the MHC molecule may be involved. Based on reiterative structures, a peptide molecule composed of only two different amino acids (Ala and His) has been produced that still retains a very high binding affinity. An 125I-radiolabeled form of this peptide has been used to demonstrate that the high binding detected is mediated by the same binding site involved in the interaction of IAd and OVA 323-339. Inhibition of Ag presentation studies further supports the immunologic relevance of the phenomena observed. Finally, we observed naturally occurring clustered binding sites in proximity of immunodominant protein regions, raising the possibility that the phenomenon might have a physiologic counterpart.     

Truncation analysis of several DR binding epitopes.

Peptide regions crucial for binding to four different DR alleles (DR1, DR2, DR5, and DR52a) have been localized in five unrelated DR binding peptides (dynorphin 1-13, sperm whale myoglobin 132-153, influenza hemagglutinin 307-319, pigeon cytochrome c 88-104, and tetanus toxoid 830-843) by testing panels of truncated analogs for DR binding. It was found that in most cases, different DR alleles recognize almost identical, albeit distinct, core regions, suggesting that different DR alleles may recognize similar structures on their peptide ligands. Furthermore, it was found that these core regions, notwithstanding their derivation from unrelated sequences, share a common structural pattern. When the sequences of several other unrelated determinants were scrutinized, the structural motif identified was present in some, but absent in other good DR binders, suggesting that good DR binding capacity of peptide molecules may be compatible with more than one single sequence pattern.     

The use of peptide analogs with improved stability and MHC binding capacity to inhibit antigen presentation in vitro and in vivo

The identification of a core region for OVA 323-339, which is critical in determining binding to IAd, has enabled us to generate a series of analog peptides in which this core region was extended at both the N and C termini with different amino acid residues. When assessed for binding capacity, several peptides were shown to have increased affinity for IAd compared with the parent sequence, and in addition, some peptides had acquired binding specificities for class II MHC haplotypes not present for OVA 323-339. These peptides were next examined for their ability to inhibit T cell responses in vitro and in vivo. The correlation between binding and the ability to inhibit T cell activation in vitro was good. However, when assessed in vivo, it was clear that high Ia binding was not sufficient in itself to define the inhibitory capacity of a given peptide. That this discrepancy was due to differences in degradation of the core-extended peptides was suggested by 1) results from an inhibition of Ag presentation assay, in which the pulse period with Ag and inhibitor was extended to 20 h; and 2) direct analysis of peptide stability by using reverse phase HPLC. Finally, by protecting the peptide from degradation with N- and C-terminal substitutions of D-amino acids, the inhibitory capacity of an unstable core-extended peptide in vitro could be greatly enhanced. These data indicate that the core extension approach may be one method by which antagonists for MHC class II molecules may be generated.     

Characterization of the specificity of peptide binding to four DR haplotypes

This study describes the establishment of a peptide-binding assay for purified, detergent-solubilized DR molecules. For each of the DR specificities and peptides studied, a unique pattern of interaction was observed. Excellent correlation was detected between the DR1-, 2-, 5-, and 52a-binding capacities and the known DR restrictions of a panel of synthetic peptides. This supports the immunologic relevance of the binding assay, and emphasizes the importance of determinant selection in defining the immune response of individuals. We have also examined the capacity of a panel of DR-restricted peptides to compete with one another for binding to DR1. The results obtained are compatible with a single peptide-binding site on DR molecules. The peptide-binding capacity of the four different DR types (DR1, DR2, DR5, and DR52a) has been further examined by testing a collection of 133 different peptides. This collection is unbiased with respect to previously known DR binding and restrictions, and includes peptides of eukaryotic, bacterial, and viral origins. It was found that: 1) approximately 15 to 35% of the peptides tested bound any given DR type; 2) DR-binding capacities appeared to correlate with each other, suggesting that different alleles of the DR isotype may recognize related structures on an Ag molecule; and 3) despite the statistical correlation between binding capacity of different DR types, approximately 50% of the peptides that were positive binders still were specific in that they could bind only one of the four DR molecules tested. Degenerate binding (i.e., binding to most or all the DR molecules tested) was detected in only a minority of the cases analyzed (approximately 25%).     

Epitopic analysis by anti-I-Ak monoclonal antibodies of I-Ak-restricted presentation of lysozyme peptides

Anti-I-A mAb were used as probes of functional epitopes for both the presentation of hen egg lysozyme (HEL) peptides to I-Ak-restricted T cell hybridomas and the direct binding of the HEL (46-61) peptide. When mAb directed to polymorphic regions of I-Ak were used as inhibitors of Ag presentation, several different patterns of inhibition were observed among T cells specific for the same HEL peptide as well as among T cells specific for different fragments of HEL. Although there appears to be a conserved usage of some TCR V beta gene segments among the T cell hybrids specific for the same HEL peptide, no correlation is evident between a single V gene usage and susceptibility to blocking of Ag presentation by a particular anti-I-Ak mAb. Several of the mAb demonstrated T cell "clonotypic blocking" of Ag presentation, whereas others blocked presentation to every T cell hybrid tested, regardless of the peptide specificity. When mAb directed to nonpolymorphic regions of the I-A molecule were tested for their ability to block Ag presentation, little or no inhibition was observed. In addition, Fab' fragments of inhibitory mAb functioned identically to their intact homologous counterparts in their ability to block Ag presentation indicating that "nonspecific" steric hindrance was not playing a major role in the inhibitions observed. When the polymorphic region-directed anti-I-A mAb were tested for their ability to block the direct binding of the lysozyme peptide HEL(46-61) to I-Ak, those mAb that block HEL presentation to all T cell hybrids were found to block the binding of this peptide. However, anti-I-A mAb that demonstrate selective inhibition of T cell hybrid stimulation during Ag presentation, i.e., those directed to polymorphic serologic specificities Ia.15 and Ia.19, do not block the binding of HEL(46-61) to I-Ak. These data indicate that functionally independent epitopes exist on the I-Ak molecule for the binding of antigenic peptides and for interaction with the TCR.     

I-Ad-binding peptides derived from unrelated protein antigens share a common structural motif

Purified Ia molecules can specifically bind many unrelated peptide Ag, and such binding appears to be a necessary, albeit not sufficient, prerequisite for the immunogenicity of the proteins from which such peptides are derived. We have recently analyzed the affect of single amino acid substitutions on the I-Ad binding of the immunogenic peptide OVA 323-339. The results obtained demonstrated the very permissive nature of Ag-Ia interaction. We also showed that unrelated peptides that are good I-Ad binders share a common structural motif and speculated that recognition of such motifs could represent a mechanism to achieve a very permissive type of interaction that yet retained some degree of specificity. In the present set of experiments we analyzed the I-Ad binding pattern of a series of overlapping peptides derived from sperm whale myoglobin (residues 102-125) and influenza hemagglutinin (residues 121-146) to determine whether the peptide regions predicted on the basis of structural similarity to be involved in I-Ad binding were in fact involved. In both cases, the I-Ad-interacting determinants were found to contain the sequence motif postulated to be important for I-Ad binding. These data support the hypothesis that I-Ad molecules recognize a large library of Ag by virtue of common structural motifs present in peptides derived from phylogenetically unrelated proteins.     

Analysis of peptide residues interacting with MHC molecule or T cell receptor. Can a peptide bind in more than one way to the same MHC molecule?

It has generally been assumed implicitly that one can define amino acid residues of a T cell antigenic determinant peptide that interact with the MHC molecule, i.e., residues that form the "agretope." However, if the same peptide can be seen in different conformations or orientations in the same MHC molecule by different T cells, then we would predict that some residues would appear to interact with the TCR of one T cell clone but with the MHC molecule as the peptide is seen by another T cell clone. To test this hypothesis, we synthesized 36 analogue peptides of an immunogenic fragment (P133-146) of sperm whale myoglobin with three different substitutions for each of 12 amino-acid residues and analyzed the role of each residue for I-Ed-binding and for activation of two Th clones, 14.1 and 14.5, specific for the peptide. The two T cell clones showed slightly different fine specificity from each other in that the truncated peptide P136-144 could stimulate 14.5 but not 14.1. The binding activity of nonstimulatory analogues to the I-Ed molecule was measured by functional inhibition analyses using truncated wild-type peptides as stimulators and nonstimulatory analogues as inhibitors. Paradoxical results were obtained that could not be explained by the peptide binding in a single way to the same I-Ed molecule. Some residues appeared to reciprocally reverse their roles for binding to I-Ed vs binding to the TCR when assessed using T-cell clone 14.5 compared to clone 14.1. These results fit the prediction of the above hypothesis and indicate the possibility that the same peptide, P133-146, can bind in more than one way to the same Ia molecule. The T cell clones, 14.1 and 14.5, appear to recognize different P133-146-I-Ed complexes in which the peptide is bound differently. Moreover, a given residue may not have a unique function of always interacting with the MHC molecule or TCR, but may change from one role to the other as it is presented to different T cells.     

Effect of pH on MHC class II-peptide interactions.

The effect of pH on class II-peptide interactions has been analyzed using several mouse (IAd, IAk, IEd, IEk) and human (DR1, DR5, DR7) MHC specificities, and eight different class II-restricted determinants. In direct binding assays, acidic conditions led to increased binding capacity for many class II-peptide combinations. IE molecules seemed to bind optimally around pH 4.5, whereas IA molecules displayed binding optima in the 5.5 to 6.5 range. In contrast, the DR molecules studied were, in most cases, affected only marginally by pH changes in the 4.5 to 7.0 range. Despite these apparent isotype-specific trends, no general rule could be formulated, because even for the same class II molecules, the binding capacity could be increased for many peptides when the binding was performed under acidic conditions, was unaffected for some, and even decreased for others. The mechanisms responsible for this complex behavior were analyzed in more detail by kinetic and equilibrium analysis of three different class II-peptide combinations (IAd/OVA 323-339, IAk/HEL 46-61, and DR1/HA 307-319). It was found that acidic pH conditions could affect both on and off rates for class II-peptide complexes. Depending on the net balance of these effects, either increases, decreases, or no effect on overall affinities at equilibrium were detected. In the case of IAd/OVA 323-339, it was also found that acidic conditions influenced the binding capacity of class II molecules by increasing the fraction of sites available for peptide binding, presumably by favoring dissociation of endogenously bound, acid-sensitive peptides.     

Structural requirements and biological significance of interactions between peptides and the major histocompatibility complex

Previous studies indicate that T cells recognize a complex between the major histocompatibility complex (MHC) restriction-element and peptide-antigen fragments. Two aspects of this complex formation are considered in this paper: (1) what is the nature of the specificity of the interactions that allows a few MHC molecules to serve as restriction elements for a large universe of antigens; and (2) what is the relative contribution of determinant selection (i.e. antigen-MHC complex formation) and T-cell repertoire in determining the capacity of an individual to respond to an antigen? By analysing single amino acid substitution analogues of a peptide antigen (Ova 325-335) as well as by analysing the structural similarities between unrelated peptides capable of binding to the same MHC molecule, we have been able to document the very permissive nature of the antigen-MHC interaction. Despite this permissiveness of binding, it is possible to define certain structural features of peptides that are associated with the capacity to bind to a particular MHC specificity. With respect to the question of the relative role of 'determinant selection' and 'holes in the T-cell repertoire' in determining immune responsiveness, we present data that suggest both mechanisms operate in concert with one another. Thus only about 30% of a collection of peptides that in sum represent the sequence of a protein molecule were found to bind to Ia. Although immunogenicity was restricted to those peptides that were capable of binding to Ia (i.e. determinant selection was operative), we found that about 40% of Ia-binding peptides were not immunogenic (i.e. there were also 'holes in the T-cell repertoire').     

MHC interaction and T cell recognition of carbohydrates and glycopeptides.

The T cell independence of complex polysaccharide Ag has suggested the possibility that carbohydrates may be incapable of T cell recognition because of a failure to interact with MHC restriction elements and/or a failure of MHC/carbohydrate complexes to interact with and be recognized by Ag-specific TCR. We have used two approaches to obtain information about T cell recognition of carbohydrate. First, we have determined the capacity of a series of oligosaccharides and glycolipids to bind a murine class II MHC molecule, IAd. No significant binding was observed with the 26 compounds tested, but the limitation to these studies was that there was a relatively limited collection of synthetic carbohydrate and glycolipid structures of limited complexity available for analysis. The second approach involved the study of the effect of glycosylation of a known peptide T cell epitope (OVA 323-339) on MHC binding of the peptide and on T cell recognition. Three patterns of effects were observed: 1) no effect on either binding or T cell recognition. This pattern was observed when the carbohydrate was located at residues removed from the core MHC-binding region. When the carbohydrate was located within the core MHC-binding regions, either 2) glycosylation destroyed both MHC binding and T cell recognition; or 3) glycosylation did not ablate MHC binding or T cell recognition. In this latter instance, there was evidence to indicate that the carbohydrate moiety was an important part of the antigenic determinant recognized by T cells.     

Constitutive competition by self proteins for antigen presentation can be overcome by receptor-enhanced uptake

The cells recognize a bimolecular ligand composed of a self Ia molecule and a fragment of foreign Ag that has been processed by an APC. The effect of self proteins on the processing and presentation of foreign Ag was examined in order to ascertain the mechanisms for competition between foreign and self Ag. How this competition can be overcome to allow an efficient immune response was also examined. Normal mouse serum proteins (NMS) compete for the processing and presentation of the foreign Ag bovine RNase by APC. This competition could have occurred at any of three levels in the APC: 1) Ag uptake, 2) Ag processing, or 3) the binding of Ag to an Ia molecule. No competition for either the uptake or the processing of RNase by self proteins could be demonstrated. However, self peptides do compete with foreign Ag by binding directly to Ia molecules, as has been shown previously. Thus, the observed inhibition by NMS of Ag presentation occurred because of competition for binding to the Ia molecule. We hypothesized that during the generation of an immune response this competition is overcome by enhanced uptake of foreign Ag. To test this, we compared the ability of NMS to compete for the presentation of RNase when it entered the APC via fluid-phase pinocytosis or through receptor-mediated uptake via the mannose receptor. When the RNase entered the APC through the mannose receptor, the ability of NMS to compete was dramatically reduced. Thus, self proteins constitutively compete for the presentation of foreign Ag at the level of binding to an Ia molecule, and this competition can be overcome by receptor-mediated uptake of the Ag.     

Fine specificity of murine class II-restricted T cell clones for synthetic peptides of influenza virus hemagglutinin. Heterogeneity of antigen interaction with the T cell and the Ia molecule

We have previously demonstrated diversity in the specificity of murine, H-2k class II-restricted, T cell clones for the hemagglutinin (HA) molecule of H3N2 influenza viruses and have mapped two T cell determinants, defined by synthetic peptides, to residues 48-68 and 118-138 of HA1. In this study we examine the nature of the determinant recognized by six distinct P48-68-specific T cell clones by using a panel of truncated synthetic peptides and substituted peptide analogs. From the peptides tested, the shortest recognized were the decapeptides, P53-62 and P54-63, which suggests that the determinant was formed from the 9 amino acids within the sequence 54-62. Asn54 was critical for recognition since P49-68 (54S) was not recognized by the T cell clones. Furthermore this peptide analog was capable of competing with P48-68 for Ag presentation, thereby suggesting that residue 54 is not involved in Ia interaction and may therefore be important for TCR interaction. Residue substitutions at position 63 also affected T cell recognition, but in a more heterogeneous fashion. Peptide analogs or mutant viruses with a single amino acid substitution at position 63 (Asp to Asn or Tyr) reduced the responses of the T cell clones to variable extents, suggesting that Asp63 may form part of overlapping T cell determinants. However since the truncated peptide P53-62 was weakly recognized, then Asp63 may not form part of the TCR or Ia interaction site, but may affect recognition through a steric or charge effect when substituted by Asn or Tyr. Ag competition experiments with the two unrelated HA peptides, P48-68 and P118-138, recognized by distinct T cell clones in the context of the same restriction element (I-Ak), showed that the peptides did not compete for Ag presentation to the relevant T cell clones, whereas a structural analog of P48-68 was a potent inhibitor. This finding is discussed in relation to the nature of the binding site for peptide Ag on the class II molecule.     

Mapping of multiple binding domains of the superantigen staphylococcal enterotoxin A for HLA.

Multiple binding sites on the staphylococcal enterotoxin A (SEA) molecule which interact with class II MHC Ag have been suggested by previous studies comparing SEA binding with that of another superantigen, toxic shock syndrome toxin-1. Using the synthetic peptide approach we have identified multiple regions of the SEA molecule which are responsible for binding to HLA Ag on Raji cells. Overlapping peptides were synthesized corresponding to the complete amino acid sequence of SEA: SEA(1-45), SEA(39-66), SEA(62-86), SEA(83-104), SEA(102-124), SEA(121-149), SEA(146-173), SEA(166-193), SEA(187-217), and SEA(211-233). Like the native SEA molecule, all of the peptides exhibited relatively high beta-sheet and low alpha-helical structure as determined by circular dichroism spectroscopy. A direct competition assay was employed with peptide blockage of 125I-SEA binding to MHC Ag. SEA(1-45), SEA(39-66), SEA(62-86), and SEA(121-149) but none of the other peptides blocked binding to Raji cells. The relative potency of the peptides in blocking SEA binding was determined with SEA(39-66) much greater than SEA(1-45) = SEA(62-86) = SEA(121-149). Peptide competition was seen at concentrations as low as 55 microM. Further, antibodies were produced to all of the peptides and tested for their ability to bind to SEA and inhibit SEA binding to HLA. Consistent with the direct inhibition of binding, antisera to SEA(1-45), SEA(39-66), and SEA(62-86) reduced the ability of SEA to bind Raji cells, whereas, antisera to the remaining peptides failed to block binding. The data suggest that the binding of the superantigen SEA to MHC molecules involves several N-terminal regions on SEA as well as an additional internal domain. This allows for the presence of multiple binding sites in an extended N-terminal region of the SEA molecule or a discontinuous binding epitope.     

Identification of T epitopes within a potential Plasmodium falciparum vaccine antigen. A study of human lymphocyte responses to repeat and nonrepeat regions of Pf155/RESA

PBMC from Melanesians who had high antibody reactivities to fusion proteins encompassing the 3' and the 5' repeat regions of the ring infected E surface antigen (Pf155/RESA), were tested for their ability to respond to synthetic and recombinant peptides representing regions of Pf155/RESA. The aim was to identify T cell epitopes within the Ag. Most of the synthetic peptides from the nonrepeat regions of Pf155/RESA were selected for study on the basis of their tendency to form amphipathic alpha-helices. Peptides representing immunodominant B cell epitopes were also tested. Three-quarters of the Melanesian donors responded to the recombinant peptides (Ag 1505 and Ag 632-100) and to the 8 x 4 mer, a synthetic peptide representative of the 3' repeat region. Whereas all the remaining eight peptides tested elicited a response in at least one donor, three peptides (M40, M42, and BTA3) representing sequences in the nonrepeat regions showed greatest promise as potentially useful T epitopes. Responses in control donors were also observed to most of the peptides but the percentage of responders was lower. T cell bulk lines specific to Ag 1505 and Ag 632-100 were established. All donors were HLA tissue typed, but no obvious correlations between responsiveness and HLA type were observed. Our results suggest that there are T cell epitopes within and outside the repeat regions of Pf155/RESA.     

On the interaction of promiscuous antigenic peptides with different DR alleles. Identification of common structural motifs.

We have investigated the interaction between DR1 molecules and the two antigenic peptides, tetanus toxoid 830-843 and hemagglutinin 307-319, previously known to bind most DR alleles (degenerate binding) and to be recognized by the same T cell clones in the context of different DR alleles (promiscuous T cell recognition). The DR1 affinity of these two peptides was compared with that of two other different T cell epitopes (pertussis toxin 30-42 and ragweed allergen Ra3 51-65). It was found that degeneracy and promiscuity were associated with high affinity interactions, whereas binding and T cell selectivity were associated with weaker interactions. Thus, the selectivity of DR-peptide interactions, as is commonly observed with the antibody molecule, appears to be inversely correlated to affinity. Several singly substituted analogs of the hemagglutinin 307-319 determinant have also been tested for capacity to bind various DR alleles (DR1, DR2, DR5, and DR7). The results obtained suggest that this determinant may bind the different DR alleles in a similar orientation. Similar conclusions were reached when the interaction between the tetanus toxoid 830-843 determinant and three different DR alleles (DR1, DR2, and DR7) was studied following the same experimental approach. When crucial DR-binding residues of the two peptides were compared, it was found that they were very similar in both chemical nature and spacing in the peptide primary structure, suggesting that the two peptides may bind DR in a very similar orientation. Finally, a putative motif has been derived and shown to be present in a majority of the DR binders tested, but only in a minority of the non-DR binding peptides.     

Class II restriction in mice to the malaria candidate vaccine ring infected erythrocyte surface antigen (RESA) as synthetic peptides or as expressed in recombinant vaccinia

The immune response to three peptides corresponding to the repeat regions of the malaria candidate vaccine ring infected E surface Ag (RESA) were studied. Both antibody responses and lymphocyte stimulation in mice injected with these peptides without carrier were found to be restricted to certain MHC class II haplotypes. Mice bearing IAk were strong responders to all three peptides. Mice bearing IAd were strong responders only to the 3' repeat peptides, the octamer and tetramer. Mice bearing Is or Iq did not respond to any repeat peptides. Remarkably, the pattern of genetic restriction of the antibody response to the entire RESA as expressed in vaccinia indicated that there were no other epitopes besides the three repeats. Because only one class II haplotype (i.e., k) out of five tested responded strongly to this peptide and only two out of five (i.e., k and d) responded to the octamer or tetramer, it may be difficult to achieve a good immune response against RESA in most or all humans.     

Specificity of peptide binding by the HLA-A2.1 molecule

The HLA-A2 molecule contains a putative peptide binding site that is bounded by two alpha-helices and a beta-pleated sheet floor. Previous studies have demonstrated that the influenza virus matrix peptide M1 55-73 can sensitize target cells for lysis by HLA-A2.1-restricted virus-immune CTL and can induce CTL that can lyse virus-infected target cells. To assess the specificity of peptide binding by the HLA-A2.1 molecule, we examined the ability of seven variant M1 peptides to be recognized by a panel of M1 55-73 peptide-specific HLA-A2.1-restricted CTL lines. The results demonstrate that five out of the seven variant M1 55-73 peptides could be recognized by A2.1-restricted M1 55-73 peptide-specific CTL lines. The two variant peptides that were not recognized by any CTL could bind to HLA-A2.1 as indicated by their ability to compete for presentation of the M1 55-73 peptide. In addition, 5 of a panel of 24 unrelated peptides tested could also compete for M1 55-73 presentation by HLA-A2.1. One peptide derived from the sequence of a rotavirus protein could sensitize HLA-A2.1+ targets for lysis by M1 55-73 peptide-specific CTL. We conclude from these studies that: 1) the HLA-A2.1 molecule can bind a broad spectrum of peptides; 2) T cells selected for the ability to recognize one peptide plus a class I molecule can actually recognize an unrelated peptide presented by that same class I molecule; and 3) a stretch of three adjacent hydrophobic amino acids may be an important common feature of peptides that can bind to HLA-A2.1.     

Random association between the peptide repertoire of A2.1 class I and several different DR class II molecules.

The interaction between synthetic peptides and A2.1 class I MHC molecules has been investigated using an inhibition of Ag presentation assay and unbiased peptide sets derived of either viral or eucaryotic origin. For the various sets, strong binding (defined as significant inhibition at the 30 micrograms/ml level) was detected in 7 to 46% of the peptides tested, with an overall frequency of 26%. A set of self-peptides derived from human beta 2 microglobulin was also included in the study. In this case, strong binding was detected in 3 of 15 peptides (20%), thus formally demonstrating a lack of self-/non-self-discrimination at the level of class I molecules. When the whole A2.1-binding database of 105 peptides thus generated was examined by sequence analysis, a significant correlation was found with a recently proposed A2.1-binding motif, whereas no particular positive or negative association was detected between the capacity to bind A2.1 and three different class II alleles (DR1, DR5, and DR7). Finally, using this approach, several peptides capable of binding both A2.1 and multiple DR alleles have been identified, suggesting possible candidates for development of peptide vaccines eliciting both class I and class II restricted responses.     

Diverse VH and VL genes are used to produce antibodies against a defined protein epitope

mAb secreting hybridomas were produced from mice hyperimmune to the model Ag tobacco mosaic virus protein. Six mAb were selected for their ability to bind synthetic peptides corresponding to amino acid residues 103-112 and 97-107 of tobacco mosaic virus protein. These mAb were analyzed for their fine specificity by measuring binding to synthetic analogs of the decapeptide, and cDNA sequences encoding the mAb V regions were determined. These analyses revealed that a wide range of different V regions are capable of binding with the same decapeptide epitope, and that these antibody sequence differences generally coincided with different binding fine specificities. This diverse antibody response with specificity for the same epitope demonstrates both the breadth of potential of the immune system and the lack of exclusivity in specific protein:protein interactions.     

Recognition of distinct adhesive sites on fibrinogen by related integrins on platelets and endothelial cells

Endothelial cells and activated platelets express integrin-type receptors responsible for adhesion to fibrinogen. We have located distinct integrin-directed endothelial cell and platelet attachment sites on immobilized fibrinogen using a combination of synthetic peptides, fibrinogen fragments, and specific anti-peptide monoclonal antibodies. Endothelial cells exclusively recognize an Arg-Gly-Asp-containing site near the C-terminus of the alpha chain (alpha residues 572-574) but fail to recognize the Arg-Gly-Asp sequence in the N-terminal region of the same chain (alpha residues 95-97). In contrast, platelets do not require either Arg-Gly-Asp sequence for binding to intact fibrinogen and are capable of recognizing, in addition to the alpha 572-574 sequence, a site at the C-terminus of the gamma chain (gamma residues 400-411). These data suggest a molecular mechanism whereby platelets and endothelial cells interact with distinct sites on the fibrinogen molecule during hemostasis and wound healing.