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.     

A single amino acid substitution in HLA-A2 can alter the selection of the cytotoxic T lymphocyte repertoire that responds to influenza virus matrix peptide 55-73

Previous studies have demonstrated that certain amino acid substitutions in the alpha two domain at positions 152 and 156 in the alpha two helix of the HLA-A2 molecule can affect presentation of the influenza virus matrix peptide M1 55-73 without abolishing binding of the M1 peptide. HLA-A2.1-restricted M1 55-73 peptide-specific CTL lines obtained from almost all HLA-A2.1+ individuals fail to recognize the M1 peptide presented by site-directed mutants of HLA-A2 that have either a Val----Ala or Val----Gln substitution at position 152 or a Leu----Trp substitution at position 156. Only one HLA-A2+ individual (donor Q66, HLA-A2,-B53,-B63) has been found who is able to generate a unique repertoire of HLA-A2-restricted M1 peptide-specific CTL that can recognize peptide presented by HLA-A2 mutants with either an Ala or Gln substitution at position 152 or a Trp substitution at position 156. These Q66 M1 peptide-specific CTL could be selected by stimulation with M1 peptide-pulsed transfectants that express the mutant HLA-A2 gene with the Trp substitution at 156. To determine if the presence of the unique CTL repertoire could be attributed to a variant HLA-A2 molecule in Q66, sequences were determined from polymerase chain reaction-amplified segments of the HLA-A2 RNA. Two different HLA-A2 genes were found expressed in Q66 cells: one is identical to HLA-A2.1 and the other is identical to HLA-A2.2F (Gln----Arg at position 43, Val----Leu at position 95, and Leu----Trp at position 156). These results demonstrate that a different CTL repertoire specific for HLA-A2 plus the M1 55-73 peptide is generated in an individual that expresses both HLA-A2.1 and HLA-A2.2F compared to individuals who express HLA-A2.1 alone, and that the unique repertoire can be selected by the presence of an HLA-A2 molecule with a single amino acid substitution at position 156.     

Differential effects of amino acid substitutions in the beta-sheet floor and alpha-2 helix of HLA-A2 on recognition by alloreactive viral peptide-specific cytotoxic T lymphocytes

Crystallographic studies of the HLA-A2 molecule have led to the assignment of a putative peptide binding site that consists of a groove with a beta-pleated sheet floor bordered by two alpha-helices. A CTL-defined variant of HLA-A2, termed HLA-A2.2F, differs from the common A2.1 molecule by three amino acids: a Leu to Trp substitution at position 156 in the alpha-2 helix, a Val to Leu substitution at position 95 in the beta-sheet floor of the groove, and a Gln to Arg substitution at position 43 in a loop outside of the groove. Another HLA-A2 variant, termed CLA, has a single Phe to Tyr substitution at position 9 that is sterically located adjacent to position 95 in the beta-sheet floor of the groove. We have determined which of the amino acid substitutions at positions 9, 43, 95, or 156 could individually affect recognition by panels of A2.1 allospecific and A2.1-restricted influenza viral matrix peptide-specific CTL lines, using a panel of site-directed mutants and CLA. Recognition by allospecific CTL lines was generally unaffected by any one of the amino acid substitutions, but was eliminated by the double substitution at positions 95 and 156. Allorecognition by some CTL lines was eliminated by a single substitution at position 9 or 95. In contrast, recognition by A2.1-restricted matrix peptide specific CTL was totally eliminated by a single substitution at position 9 or 156. The substitution at position 43 in a loop away from the peptide binding groove had no effect on allorecognition or matrix peptide recognition. These results indicate that amino acid residues in the floor or alpha-2 helical wall of the peptide binding groove of the HLA-A2 molecule can differentially affect allorecognition and viral peptide recognition.     

Presentation of three different viral peptides, HTLV-1 Tax, HCMV gB, and influenza virus M1, is determined by common structural features of the HLA-A2.1 molecule.

To determine whether similar or dissimilar molecular features of class I molecules are involved in the presentation of structurally distinct peptides, we have investigated the influence of different pockets of the HLA-A2.1 molecule on the presentation of three different viral peptides. HTLV-I Tax peptide 12-19, HCMV gB 619-628, and influenza M1 58-66 are minimal peptides that induce HLA-A2.1-restricted noncross-reactive CTL. A detailed analysis of the structural features of HLA-A2.1 that are involved in peptide presentation was undertaken using a panel of 11 HLA-A2 mutants with single amino acid substitutions within pockets present in the peptide binding site. Nine of the 11 mutants affected presentation of each of the three peptides, whereas the other two mutants had negative effects on presentation of only two of these viral peptides. These results indicate that common structural features in HLA-A2 determine the binding of different peptides, and help to provide a plausible explanation for how structurally diverse peptides bind to HLA-A2.     

Alloreactive cytotoxic T-lymphocyte-defined HLA-B7 subtypes differ in peptide antigen presentation

We investigated T-cell-defined HLA-B7 subtypes using cDNA sequencing, analysis of bound peptides, and reactivity with a panel of alloreactive cytotoxic T-lymphocyte (CTL) clones. Three subtypes (HLA-B*0702, HLA-B*0703, and HLA-B*0705) differ in nucleotide and predicted amino acid sequence. CTL reactivity and pooled peptide sequencing show that these three HLA-B7 subtypes bind distinct but overlapping sets of peptides. In particular B*0702 expresses D pocket residue Asp 114 and binds peptides with P3 Arg, whereas B*0705 expresses D pocket residue Asn 114 and binds peptides with P3 Ala, Leu, and Met. Consistent with different peptide-binding specificities, three alloreactive CTL differentiate between cells expressing B*0702, B*0703, and B*0705 by detecting specific peptide/HLA-B7 complexes. In contrast, three other T-cell-defined HLA-B7 subtypes are identical to HLA-B*0702. The B*0702-expressing cell lines are differentiated by two of ten CTL clones. One CTL clone differentiates B*0702-expressing cells by their ability to present peptide antigen. Thus differences in peptide presentation can explain differential CTL recognition of cell lines expressing structurally identical and variant HLA-B7.     

Construction and immunogenicity prediction of Plasmodiumfalciparum CTL epitope minigene vaccine

The minigenes encoding Plasmodium falciparum CTL epitopes restricted to human MHC class I molecular HLA-A2 and HLA-B51, which were both at high frequency among Chinese population, were constructed as mono-epitope CTL vaccines named pcDNA3.1/frand pcDNA3.1/ sh. The minigenes of the two epitopes were then tandem linked to form a dimeric CTL epitope minigene recombinant vaccine. After DNA transfection, the epitope minigenes were expressed respectively in two human cell lines, each bearing one MHC class I molecule named CIR/HLA-A2.1 and K562/HLA-B51. The intraceliular expression of the CTL epitope minigenes not only enhanced the stability of HLA-A2.1 and HLA-B51 molecules but also increased the assemblage of MHC class I molecules on cell surfaces, which testified the specific process and presentation of those endogenous expressed epitopes. For the cells transfected with the dimeric minigene encoding two tandem linked epitopes, the expression and presentation of each epitope were also detected on cell membran     

Changes at the floor of the peptide-binding groove induce a strong preference for proline at position 3 of the bound peptide: molecular dynamics simulations of HLA-A*0217

We report on molecular dynamics simulations of major histocompatibility complex (MHC)-peptide complexes. Class I MHC molecules play an important role in cellular immunity by presenting antigenic peptides to cytotoxic T cells. Pockets in the peptide-binding groove of MHC molecules accommodate anchor side chains of the bound peptide. Amino acid substitutions in MHC affect differences in the peptide-anchor motifs. HLA-A*0217, human MHC class I molecule, differs from HLA-A*0201 only by three amino acid residues substitutions (positions 95, 97, and 99) at the floor of the peptide-binding groove. A*0217 showed a strong preference for Pro at position 3 (p3) and accepted Phe at p9 of its peptide ligands, but these preferences have not been found in other HLA-A2 ligands. To reveal the structural mechanism of these observations, the A*0217-peptide complexes were simulated by 1000 ps molecular dynamics at 300 K with explicit solvent molecules and compared with those of the A*0201-peptide complexes. We examined the distances between the anchor side chain of the bound peptide and the pocket, and the rms fluctuations of the bound peptides and the HLA molecules. On the basis of the results from our simulations, we propose that Pro at p3 serves as an optimum residue to lock the dominant anchor residue (p9) tightly into pocket F and to hold the peptide in the binding groove, rather than a secondary anchor residue fitting optimally the complementary pocket. We also found that Phe at p9 is used to occupy the space created by replacements of three amino acid residues at the floor within the groove. These findings would provide a novel understanding in the peptide-binding motifs of class I MHC molecules.     

Construction and immunogenicity prediction of Plasmodium falciparum CTL epitope minigene vaccine

The minigenes encoding Plasmodiumfalciparum CTL epitopes restricted to human MHC class I molecular HLA-A2 and HLA-B51, which were both at high frequency among Chinese population, were constructed as mono-epitope CTL vaccines named pcDNA3.1/tr and pcDNA3.1/sh. The minigenes of the two epitopes were then tandem linked to form a dimeric CTL epitope minigene recombinant vaccine. After DNA transfection, the epitope minigenes were expressed respectively in two human cell lines, each bearing one MHC class I molecule named CIR/HLA-A2.1 and K562/HLA-B51. The intracellular expression of the CTL epitope minigenes not only enhanced the stability of HLA-A2.1 and HLA-B51 molecules but also increased the assemblage of MHC class I molecules on cell surfaces, which testified the specific process and presentation of those endogenous expressed epitopes. For the cells transfected with the dimeric minigene encoding two tandem linked epitopes, the expression and presentation of each epitope were also detected on cell membranes that bore different MHC class I molecules. It meant that the adjacency of the two CTL epitopes did not interfere with the specific process and presentation of each epitope. Compared with the ordinary CTL studies that inoculated synthesized epitope peptides with peripheral blood cells, this work aimed to process the epitopes directly inside HLA class I allele specific human cells, and thus theoretically imitated the same procedurein vivo. It was also an economical way to predict the immunogenicity of CTL epitopes at an early stage especially in laboratories with limited financial resource.     

The structure and stability of an HLA-A*0201/octameric tax peptide complex with an empty conserved peptide-N-terminal binding site

The crystal structure of the human class I MHC molecule HLA-A2 complexed with of an octameric peptide, Tax8 (LFGYPVYV), from human T cell lymphotrophic virus-1 (HTLV-1) has been determined. This structure is compared with a newly refined, higher resolution (1.8 A) structure of HLA-A2 complexed with the nonameric Tax9 peptide (LLFGYPVYV) with one more N-terminal residue. Despite the absence of a peptide residue (P1) bound in the conserved N-terminal peptide-binding pocket of the Tax8/HLA-A2 complex, the structures of the two complexes are essentially identical. Water molecules in the Tax8 complex replace the terminal amino group of the Tax9 peptide and mediate a network of hydrogen bonds among the secondary structural elements at that end of the peptide-binding groove. Thermal denaturation measurements indicate that the Tax8 complex is much less stable, DeltaTm = 16 degrees C, than the Tax9 complex, but both can sensitize target cells for lysis by some Tax-specific CTL from HTLV-1 infected individuals. The absence of a P1 peptide residue is thus not enough to prevent formation of a "closed conformation" of the peptide-binding site. TCR affinity measurements and cytotoxic T cell assays indicate that the Tax8/HLA-A2 complex does not functionally cross-react with the A6-TCR-bearing T cell clone specific for Tax9/HLA-A2 complexes.     

Overlapping epitopes that are recognized by CD8+ HLA class I-restricted and CD4+ class II-restricted cytotoxic T lymphocytes are contained within an influenza nucleoprotein peptide.

Viral epitopes that are recognized by both HLA class I-restricted and class II-restricted T cells have been defined for a type A influenza virus nucleoprotein (NP) peptide. CD8+ and CD4+ CTL lines have been generated against a synthetic peptide encompassing residues 335 to 349 of NP that are restricted by HLA-B37 and HLA-DQw5, respectively. Both of these CTL populations were capable of specifically lysing influenza A virus-infected targets, indicating that a naturally processed NP peptide(s) was being mimicked by the NP (335-349) peptide. Amino acid residues that are critical for recognition of this NP determinant in the context of HLA-B37 and HLA-DQw5 were investigated by the use of panels of truncated and alanine-substituted NP peptides. The results demonstrate that: 1) truncations in the amino- or carboxy-terminal ends differentially affect CD8+ and CD4+ CTL recognition; 2) the NP (335-349) sequence contains two octapeptide epitopes that share a core of six amino acid residues (NP 338-343); and 3) alanine substitutions at five of these residues abrogated recognition by at least one of the CD8+ and CD4+ CTL lines. Thus, these class I- and class II-restricted CTL lines recognize similar but distinct epitopes, and different structural features of the NP peptide are required for presentation by HLA-B37 and HLA-DQw5. Comparison of the amino acid sequences of the NP peptide presented by HLA-B37 and HLA-DQw5 with other peptides known to be presented by both class I and class II molecules revealed a common motif among these peptides.     

MHC recognition by hapten-specific HLA-A2-restricted CD8+ CTL

T cell recognition by peptide-specific alphabeta TCRs involves not only recognition of the peptide, but also recognition of multiple molecular features on the surface of the MHC molecule to which the peptide has been bound. We have previously shown that TCRs that are specific for five different peptides presented by HLA-A2 recognize similar molecular features on the surface of the alpha1 and alpha2 helices of the HLA-A2 molecule. We next asked whether these same molecular features of the HLA-A2 molecule would be recognized by hapten-specific HLA-A2-restricted TCRs, given that hapten-specific T cells frequently show reduced MHC dependence/restriction. The results show that a panel of CD8+ CTL that are specific for the hapten DNP bound to two different peptides presented by HLA-A2 do the following: 1) show stringent MHC restriction, and 2) are largely affected by the same mutations on the HLA-A2 molecule that affected recognition by peptide-specific CTL. A small subset of this panel of CD8+ CTL can recognize a mutant HLA-A2 molecule in the absence of hapten. These data suggest that TCR recognition of a divergent repertoire of ligands presented by HLA-A2 is largely dependent upon common structural elements in the central portion of the peptide-binding site.     

Role of binding pockets for amino-terminal peptide residues in HLA-B27 allorecognition.

The peptide binding site of HLA-B27 and other class I Ag consists of a series of pockets that bind peptide side chains. Two of these pockets interact with the amino-terminal peptide residue (pocket A) and with the highly conserved second residue (pocket B). In this study, the role of pockets A and B in HLA-B27-specific T cell allorecognition has been analyzed. Four HLA-B27 mutants with single or double changes in pocket B (24T----A, 45E----M, 67C----V, and 24,67T,C----A,V) and three mutants with single changes in pocket A (163E----T, 167W----S, and 171Y----H) were constructed by site-directed mutagenesis and expressed in HMy2.C1R cells after DNA-mediated gene transfer. These transfectants were used as target cells in cytotoxicity assays with a series of HLA-B27-specific CTL. All the mutations analyzed affected allorecognition by a significant proportion of the CTL tested, but no single change abrogated recognition by all CTL. The global effects of each mutation on allorecognition were comparable to one another, except for the effect of the change at position 67, which was smaller. The behavior of individual CTL with the mutants was very diverse, ranging from CTL that did not recognize most of the mutants to CTL recognizing all of them. Thus, some alloreactive CTL can withstand drastic alterations in pockets A and B. Two CTL showed heteroclytic effects towards the V67 and M45 mutants. CTL behavior with the H171 mutant was closely parallel to that with the B*2703 subtype, having a single Y----H change at position 59. This parallelism correlates with the similar role of Tyr59 and Tyr171 in establishing hydrogen bonds with the amino termini of HLA-B27-bound peptides. The results demonstrate that altering the structure of pockets that interact with the amino-terminal first and second residues of HLA-B27-bound peptides significantly affects recognition by alloreactive CTL, and they strongly suggest widespread peptide involvement in HLA-B27 allorecognition.     

Assessment of immunogenicity of human Melan-A peptide analogues in HLA-A*0201/Kb transgenic mice

Previous studies have shown that substitution of single amino acid residues in human Melan-A immunodominant peptides Melan-A27-35 and Melan-A26-35 greatly improved their binding and the stability of peptide/HLA-A*0201 complexes. In particular, one Melan-A peptide analogue was more efficient in the generation of Melan-A peptide-specific and melanoma-reactive CTL than its parental peptide in vitro from human PBL. In this study, we analyzed the in vivo immunogenicity of Melan-A natural peptides and their analogues in HLA-A*0201/Kb transgenic mice. We found that two human Melan-A natural peptides, Melan-A26-35 and Melan-A27-35, were relatively weak immunogens, whereas several Melan-A peptide analogues were potent immunogens for in vivo CTL priming. In addition, induced Melan-A peptide-specific mouse CTL cross-recognized natural Melan-A peptides and their analogues. More interestingly, these mouse CTL were also able to lyse human melanoma cell lines in vitro in a HLA-A*0201-restricted, Melan-A-specific manner. Our results indicate that the HLA-A*0201/Kb transgenic mouse is a useful animal model to perform preclinical testing of potential cancer vaccines, and that Melan-A peptide analogues are attractive candidates for melanoma immunotherapy.     

MHC allele-specific molecular features determine peptide/HLA-A2 conformations that are recognized by HLA-A2-restricted T cell receptors

The structures of alphabeta TCRs bound to complexes of class I MHC molecules and peptide show that the TCRs make multiple contacts with the alpha1 and alpha2 helixes of the MHC. Previously we have shown that the A6 TCR in complex with the HLA-A2/Tax peptide has 15 contact sites on HLA-A2. Single amino acid mutagenesis of these contact sites demonstrated that mutation of only three amino acids clustered on the alpha1 helix (R65, K66, A69) disrupted recognition by the A6 TCR. In the present study we have asked whether TCRs that recognize four other peptides presented by HLA-A2 interact with the MHC in identical, similar, or different patterns as the A6 TCR. Mutants K66A and Q155A had the highest frequency of negative effects on lysis. A subset of peptide-specific CTL also selectively recognized mutants K66A or Q155A in the absence of exogenous cognate peptides, indicating that these mutations affected the presentation of endogenous peptide/HLA-A2 complexes. These findings suggest that most HLA-A2-restricted TCRs recognize surfaces on the HLA-A2/peptide complex that are dependent upon the side chains of K66 and Q155 in the central portion of the peptide binding groove. Crystallographic structures of several peptide/HLA-A2 structures have shown that the side chains of these critical amino acids that make contact with the A6 TCR also contact the bound peptide. Collectively, our results indicate that the generalized effects of changes at these critical amino acids are probably due to the fact that they can be directly contacted by TCRs as well as influence the binding and presentation of the bound peptides.     

Influenza A-specific, HLA-A2.1-restricted cytotoxic T lymphocytes from HLA-A2.1 transgenic mice recognize fragments of the M1 protein.

Previous studies have indicated that in transgenic mice expressing human class I MHC molecules, it is difficult to demonstrate a significant CTL response to a viral Ag in the context of the transgenic molecule. In this paper, a procedure is reported for the isolation of influenza-specific murine CTL restricted by the human class I molecule HLA-A2.1. The principal specificity of such CTL is for a fragment of the influenza M1 protein that has been previously shown to be immunodominant for human HLA-A2.1-restricted CTL. CTL of this specificity were also established through the use of peptide-pulsed rather than virus-infected stimulators. The dependence of murine CTL recognition upon peptide length and HLA-A2 structure was established to be similar to that previously reported for human CTL. However, the fine specificity of CTL maintained on virus-infected stimulators was somewhat different from that of CTL maintained with M1 peptide. This suggests that differences in surface density or peptide structure between peptide-pulsed and virus-infected stimulators may result in the outgrowth of T cells with different receptor structures. The immunodominance of the M1 peptide determinant in both mice and humans suggests that species-specific differences in TCR structure, Ag-processing systems, and self-tolerance are of less importance than limitations on the ability of antigenic peptides to bind to appropriate class I molecules. These results thus establish the utility of the transgenic system for the identification of human class I MHC-restricted T cell epitopes.     

Amino acid residues 56 to 69 of HLA-A2 specify an antigenic determinant shared by HLA-A2 and HLA-B17

The mouse monoclonal antibody MA2.1 was previously used to define an epitope shared by native HLA-A2 and HLA-B17 molecules and amino acid sequence comparison of nine HLA-A,B,C molecules identified residues 62 to 65 as the region most likely to form this epitope. An unabsorbed rabbit antiserum raised against a peptide corresponding to residues 56 to 69 of HLA-A2 gives highly specific reactions with HLA-A2 and HLA-B17 heavy chains in Western blots. No interactions with native HLA-A2 and B17 molecules were detected in a variety of assays. Although the topographic relationship between the epitopes recognized by the rabbit antiserum and the monoclonal antibody could not be determined, the results show that residues 56 to 69 of HLA-A2 can form epitopes with specificity for HLA-A2 and HLA-B17.     

Identification by site-directed mutagenesis of amino acid residues contributing to serologic and CTL-defined epitope differences between HLA-A2.1 and HLA-A2.3

Site-directed mutagenesis of HLA-A2.1 has been used to identify the amino acid substitutions in HLA-A2.3 that are responsible for the lack of recognition of the latter molecule by the HLA-A2/A28 specific antibody, CR11-351, and by HLA-A2.1 specific CTL. Three genes were constructed that encoded HLA-A2 derivatives containing one of the amino acids known to occur in HLA-A2.3: Thr for Ala149, Glu for Val152, and Trp for Leu156. Three additional genes were constructed that encoded the different possible combinations of two amino acid substitutions at these residues. Finally, a gene encoding all three substitutions and equivalent to HLA-A2.3 was constructed. These genes were transfected into the class I negative, human cell line Hmy2.C1R. Analysis of this panel of cells revealed that recognition by the antibody CR11-351 was completely lost when Thr was substituted for Ala149, whereas substitutions at amino acids 152 and 156, either singly or in combination, had no effect on the binding of this antibody. The epitopes recognized by the allogeneic and xenogeneic HLA-A2.1 specific CTL clones used in this study were all affected by either one or two amino acid substitutions. Of those epitopes sensitive to single amino acid changes, none were affected by the substitution of Thr for Ala149, whereas all of them were affected by at least one of the substitutions of Glu for Val 152 or Trp for Leu156. Overall, amino acid residue 152 exerted a stronger effect on the epitopes recognized by HLA-A2.1 specific CTL than did residue 156. Of those epitopes affected only by multiple amino acid substitutions, double substitutions at residues 149 and 152 or at 152 and 156 resulted in a loss of recognition, whereas a mutant with substitutions at residues 149 and 156 was recognized normally. This reemphasizes the importance of residue 152 and indicates that residue 149 can affect epitope formation in conjunction with another amino acid substitution. These results are discussed in the context of current models for the recognition of alloantigens and in light of the recently published three-dimensional structure of the HLA-A2.1 molecule.     

Cytotoxic T lymphocyte-defined epitope differences between HLA-A2.1 and HLA-A2.2 map to two distinct regions of the molecule

Hemi-exon shuffling and site-directed mutagenesis have been used to determine which amino acid differences between HLA-A2.1 and HLA-A2.2 alter the CTL-defined epitopes on these two molecules. Two genes were constructed that encode novel molecules in which the effect of amino acid differences at residues 9, 43, and 95, or at residue 156 could be separately evaluated. Using both human and murine CTL that were specific for either HLA-A2.1 or HLA-A2.2, four types of epitopes were identified: 1) epitopes that were insensitive to substitutions at either residues 9, 43, and 95, or residue 156 but were lost when all four positions were changed; 2) epitopes that were dependent on the residues 9, 43, 95, but not residue 156; 3) epitopes that were dependent on residue 156, but not amino acid residues 9, 43, and 95; and 4) epitopes that were dependent on residues 9, 43, and 95, as well as amino acid residue 156. Overall, there was a roughly equal distribution of clones recognizing each of these types of epitopes. Additional molecules were constructed by hemi-exon shuffling between the HLA-A2.2 and HLA-A2.3 genes, and by site-directed mutagenesis, to analyze the epitopes recognized by two HLA-A2.2/A2.1 cross-reactive murine CTL that do not recognize HLA-A2.3. Although the epitopes recognized by these CTL were unaffected by changes occurring at residues 9, 43, and 95, or at residues 149, 152, and 156 alone, simultaneous changes in both of these regions acted in concert to destroy the epitopes. Both of the CTL recognized epitopes that were lost when substitutions were made at residues 9, 43, 95, 149, and 152. The epitope recognized by one of the CTL was also destroyed by the substitution of residues 9, 43, 95, 152, and 156. Overall, these results indicate that residues 9, 43, and 95, as well as residues in the alpha-helical region of the molecule, are all capable of contributing to the definition of the epitopes recognized by HLA-A2.1- and HLA-A2.2-specific CTL. They further indicate that some epitopes can be mapped to a particular region of the molecule, whereas other epitopes are formed through a complex interaction of residues in distant regions of the molecule.     

A cluster of mutations in HLA-A2 α2 helix abolishes peptide recognition by T cells

In order to investigate the regions of HLA-A2 that control peptide-specific cytotoxic T lymphocyte (CTL) recognition, 37 HLA-A2 genes coding for 50 point mutations that span the 2 helix were synthesized by the technique of saturation mutagenesis. Twenty-nine of these genes, which code for 41 point mutations, were transfected into C1R cells and used as targets in cytotoxicity assays, in the presence of influenza-A matrix peptide 58–68 with specific CTL as effectors. All the transfectants were recognized fully by matrix peptide-specific CTL apart from those with amino acid substitutions at positions 152, 154, 155, 156, or 161, which led to a total loss of recognition and those with mutations at residue 27 or a double mutation at 138 and 150, which were recognized in an intermediate manner. The clustering of the crucial residues that emerges may reflect direct interaction of their side-chains with peptide or the CTL receptor. Address correspondence and offprint requests to: R. J. Moots.