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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.

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.

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.

The authors review their recent research involving the generation of cytotoxic T lymphocytes (CTL) directed against HLA-DR antigens. A mouse anti-human xenogeneic system first suggested that HLA-DR antigens could be recognized by CTL. Human allogeneic CTL specific for HLA-DR6 were generated and found to be OKT4+. The fact that these CTL were OKT4+ while anti HLA-A,B CTL were OKT8+ suggested that these T cell surface antigens may be involved in MHC antigen recognition; ie, they may be part of the T cell receptor. These OKT4+, HLA-DR specific CTL were further used to generate monoclonal antibodies (1) which block cytolysis and define novel antigens involved in the CTL-target interaction and (2) which define an antigenic complex on alloantigen activated T cells.

HLA-A2.1 and HLA-A2.3, which differ from one another at residues 149, 152, and 156, can be distinguished by the mAb CR11-351 and many allogeneic and xenogeneic CTL. Site-directed mutagenesis was used to incorporate several different amino acid substitutions at each of these positions in HLA-A2.1 to evaluate their relative importance to serologic and CTL-defined epitopes. Recognition by mAb CR11-351 was completely lost when Thr but not Pro was substituted for Ala149. A model to explain this result based on the 3-dimensional structure of HLA-A2.1 is presented. In screening eight other mAb, only the substitutions of Pro for Val152 or Gly for Leu156 led to the loss of mAb binding. Because other non-conservative substitutions at these same positions had no effect, these results suggest that the loss of serologic epitopes is in many cases due to a more indirect effect on molecular conformation. Specificity analysis using 28 HLA-A2.1-specific alloreactive and xenoreactive CTL clones showed 19 distinct patterns of recognition. The epitopes recognized by alloreactive CTL clones demonstrated a pronounced effect by all substitutions at residue 152, including the very conservation substitution of Ala for Val. Overall, the most disruptive substitution at amino acid residue 152 was Pro, followed by Glu, Gln, and then Ala. In contrast, substitutions at 156 had little or no effect on allogeneic CTL recognition, and most clones tolerated either Gly, Ser, or Trp at this position. Similar results were seen using a panel of murine HLA-A2.1-specific CTL clones, except that substitutions at position 156 had a greater effect. The most disruptive substitution was Trp, followed by Ser and then Gly. In addition, when assessed on the entire panel of CTL, the effects of Glu and Gln substitutions at position 152 demonstrated that the introduction of a charge difference is no more disruptive than a comparable change in side chain structure that does not alter charge. Taken together, these results indicate that the effect of amino acid replacements at positions 152 and 156 on CTL-defined epitopes depends strongly on the nature of the substitution. Thus, considerable caution must be exercised in evaluating the significance of particular positions on the basis of single mutations. Nonetheless, the more extensive analysis conducted here indicates that there are differences among residues in the class I Ag "binding pocket," with residue 152 playing a relatively more important role in formation of allogeneic CTL-defined epitopes than residue 156.

OKT4+, HLA-DR-specific CTL were cloned by limiting dilution, and two clones were evaluated. One clone, B8, specifically recognized DR6 antigens, whereas another cloned, C6, recognized an Ia-like determinant on some DR 3, 5, and 6 target cells. Both clones were OKT3+, OKT4+, and OKT8-, and their cytolysis could be blocked by OKT3 and OKT4, but not OKT8, antibodies. A panel of monoclonal antibodies that recognize DR molecules blocked target cell recognition by these OKT+ CTL. Although cloned B8 recognized a DR6-specific determinant, clone C6 appeared to recognize a supratypic determinant that may be common to some DR molecules (e.g., MT2) or possibly another human Ia-like antigen (e.g., SB). The availability of OKT+ CTL clones should help to dissect the Ia-like antigens recognized by human T cells.

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.

We describe the function and cell distribution of two novel cell surface antigens, L24 and L25. These antigens are broadly distributed on human lymphocytes. Monoclonal antibodies specific for these molecules block lysis by Class I- and II-specific cytotoxic T lymphocytes, but do not affect any other T cell functions tested. Anti-L24 antibody immunoprecipitates a molecule composed of two disulfide-linked monomers of 140 kd each. Anti-L25 antibody immunoprecipitates three proteins of 150, 85, and 75 kd. The study of these and other function associated molecules may provide insight into mechanisms of cytotoxic T lymphocyte recognition and/or function.