Multiple regions of HLA-DR beta 1 chains determine polymorphic epitopes recognized by monoclonal antibodies

To investigate the locations of antibody binding epitopes on HLA class II molecules, four DR4/7 beta 1 hybrid cDNA were constructed by exchanging the DNA encoding the NH2-terminal portions (amino acids 1 to 40) or the COOH-terminal portions (amino acids 41 to 94) of the first domains of DR4 beta 1- and DR7 beta 1-chains, in association with DNA encoding either the DR4 beta 1 or DR7 beta 1 second domains. Transfectants expressing a DR alpha cDNA and a wild-type DR4 beta 1 or DR7 beta 1 cDNA or one of four hybrid DR4/7 beta 1 cDNA were produced, and the binding to the transfectants of anticlass II mAb, which detect polymorphic epitopes on either DR4 or DR7 molecules, was analyzed. Four different patterns of mAb binding to the transfectants were observed, indicating that multiple regions of DR beta 1-chains play the predominant roles in the contributions of these chains to polymorphic epitopes recognized by mAb on intact molecules. The relevant regions of these chains and the number of mAb that recognize the associated polymorphic epitopes are: 1) the COOH-terminal portion of the first domain of DR4 beta 1; a DR4-specific mAb, 2) the NH2-terminal portion of the first domain of DR7 beta 1; two mAb, including a DR7-specific mAb, 3) the NH2-terminal portion of the first domain of DR4 beta 1; seven mAb, and 4) the second domain of DR4 beta 1; one mAb.     

Mapping of distinct serologic and T cell recognition epitopes on an HLA-DR beta-chain.

A new DR beta-chain allele is defined that is identical to the previously described DR6b molecule except for the first hyperpolymorphic region, where the new allele displays the same polymorphisms found on DR8 and DR12 genes. Two distinct epitopes have been mapped on this new allele. The polymorphism in common with DRw8 and DRw12 is recognized by mAb GS313-9D11. However, alloreactive T cell clones specific for DR6b cells (Dw9) recognize this allele, whereas Dw8-specific T cell clones do not. The mAb determinant maps to the first beta-sheet and probably involves a polymorphic residue lying outside the helix. The binding of mAb 9D11 to this region does not interfere with TCR binding. Alloreactive T cell recognition is associated with polymorphisms located predominantly on the alpha-helical portion of the molecule.     

Mutations in the third, but not the first or second, hypervariable regions of DR(beta 1*0101) eliminate DR1-restricted recognition of a pertussis toxin peptide.

We have examined the role of 12 polymorphic residues of the beta-chain of the HLA-DR1 class II molecule in T cell recognition of an epitope of pertussis toxin. Murine L cell transfectants expressing wild-type or mutant DR1 molecules (containing single amino acid substitutions in DR(beta 1*0101)) were used as APC in proliferation assays involving nine DR1-restricted T cell clones specific for peptide 30-42 of pertussis toxin. Four different patterns of recognition of the mutants were found among the pertussis-specific clones. Residues in the third hypervariable region (HVR) of DR(beta 1*0101) are critically important for all the T cell clones; amino acid substitutions at positions 70 and 74 abrogated recognition by all of the T cell clones, and substitutions at positions 67 and 71 eliminated recognition by most of the clones. In contrast, most single amino acid substitutions in the first and second HVR, predicted to be located in the floor of the peptide binding groove, had little or no effect on the proliferative responses of these clones. However, the involvement of beta-chain first and second HVR residues was demonstrated by the inability of transfectants expressing wild-type DR(beta 1*0404) (DR4Dw14) or DR(beta 1*1402) (DR6Dw16) to present peptide to these clones. These beta-chains have completely different first and second HVR compared with DR(alpha,beta 1*0101) although the third HVR are identical. These results illustrate the functional importance of third HVR residues of DR(beta 1*0101) and allow definition of the molecular interactions of the DR1 molecule with the 30-42 peptide.     

Molecular characterization of serologic recognition sites in the human HLA-A2 molecule

The localization of the amino acid residues involved in the serologic specificity of the HLA-A2 molecule has been investigated using a combination of site-directed mutagenesis, DNA-mediated gene transfer, indirect immunofluorescence and flow cytometry techniques. Synthetic oligonucleotides were designed to introduce individual and combined amino acid substitutions in both the alpha 1 (positions 9, 43, and the highly polymorphic cluster of residues from aa 62 to 83) and alpha 2 (positions 107, 152, and 156) domains to investigate the effect of the specific mutation on the recognition of the molecule at the surface of transfected human and mouse cell lines by a panel of mAb that recognize monomorphic or polymorphic determinants in MHC class I molecules. At least three non-overlapping serologic epitopes were identified. Mutations in the highly polymorphic region at aa 62 to 66 completely eliminated binding of mAb MA2.1 (A2/B17 cross-reactive). Mutation at position 107 resulted in complete loss of binding of the A2/Aw69-specific mAb PA2.1 and MA2.2 and partial loss of mAb BB7.2 binding. The recognition by other allotypic mAbs was not affected by these mutations and they therefore represent at least a third serologic epitope. Mutations at positions 152 and 156, known to be important for T cell recognition, did not affect serologic recognition. Introduction of residues of HLA-B7 origin in the polymorphic segment spanning aa 70 to 80 created a molecule carrying the -Bw6 supertypic determinant as demonstrated by mAb SFR8-B6 binding.     

Crystal structure of staphylococcal enterotoxin I (SEI) in complex with a human major histocompatibility complex class II molecule

Superantigens are bacterial or viral proteins that elicit massive T cell activation through simultaneous binding to major histocompatibility complex (MHC) class II and T cell receptors. This activation results in uncontrolled release of inflammatory cytokines, causing toxic shock. A remarkable property of superantigens, which distinguishes them from T cell receptors, is their ability to interact with multiple MHC class II alleles independently of MHC-bound peptide. Previous crystallographic studies have shown that staphylococcal and streptococcal superantigens belonging to the zinc family bind to a high affinity site on the class II beta-chain. However, the basis for promiscuous MHC recognition by zinc-dependent superantigens is not obvious, because the beta-chain is polymorphic and the MHC-bound peptide forms part of the binding interface. To understand how zinc-dependent superantigens recognize MHC, we determined the crystal structure, at 2.0 A resolution, of staphylococcal enterotoxin I bound to the human class II molecule HLA-DR1 bearing a peptide from influenza hemagglutinin. Interactions between the superantigen and DR1 beta-chain are mediated by a zinc ion, and 22% of the buried surface of peptide.MHC is contributed by the peptide. Comparison of the staphylococcal enterotoxin I.peptide.DR1 structure with ones determined previously revealed that zinc-dependent superantigens achieve promiscuous binding to MHC by targeting conservatively substituted residues of the polymorphic beta-chain. Additionally, these superantigens circumvent peptide specificity by engaging MHC-bound peptides at their conformationally conserved N-terminal regions while minimizing sequence-specific interactions with peptide residues to enhance cross-reactivity.     

Molecular variation of human major histocompatibility complex DQw3 beta-chains

Histocompatibility leukocyte antigen DQ molecules exhibit polymorphism of both DQ alpha- and beta-chains. Histocompatibility leukocyte antigen-DQw3 is associated with both DR4 and DR5 and can be further subdivided by reactivity with the monoclonal antibody TA10. To determine the molecular nature of the DQ polymorphic alleles associated with the DR4 haplotype, we have sequenced and analyzed DQ alpha and beta cDNA clones obtained from a DR4, Dw4, DQw3 cell line which is TA10-positive. The DQ alpha-chain sequence was identical to previously published sequences from the DR4 haplotype, but the DQ beta sequence differed from published DR4-DQ beta sequences obtained from DQw3-positive TA10-negative cell lines by eight amino acids, six of which were located in the beta 1 domain. Thus, the TA10 serologic determinants reside on the DQ beta-chain. A TA10-specific oligonucleotide probe was constructed based on the DQ beta sequence, and its specificity was confirmed in a panel of TA10-positive and TA10-negative cell lines. An additional band was observed in Southern blotting experiments which may indicate a donor sequence for gene conversion.     

A newly characterized HLA-DP beta-chain allele. Evidence for DP beta heterogeneity within the DPw4 specificity

cDNA clones corresponding to the DPw4 alpha- and DPw4 beta-chains were isolated from a cDNA library prepared from a DPw4 homozygous cell line, their nucleotide sequences were determined, and the corresponding amino acid sequences were deduced. This DPw4 alpha-chain is identical to the conserved DP alpha-chains from DPw4 and DPw2 haplotypes, although the DPw4 beta-chain (referred to as DPw4b beta) differs from all reported DP beta-chain sequences. The DPw4b beta-chain differs from the reported DPw4 beta sequence (referred to as DPw4a beta) at three amino acid positions in the first domain (36, 55, and 56). The DPw4b beta-chain sequence differs from the DPw2 beta-chain sequence only at position 69 in the first domain, suggesting that the lysine at position 69 in DPw4b beta and the glutamic acid at position 69 in DPw2 beta contribute to the epitopes that define "DPw4-ness" and "DPw2-ness," respectively. In addition, the patterns of sequence identities and differences among the DPw4b beta-, DPw4a beta-, DPw2 beta-, and DPw3 beta-chains suggest that the DPw4b beta sequence arose via a gene conversion event or a point mutation. The I-LR1 mAb, which was previously found to bind only to DPw2, DPw3, and DR5 molecules, binds to an L cell transfectant expressing the DPw4 alpha:DPw4b beta molecule. The DPw4b beta sequence provides the first evidence for structural heterogeneity within the DPw4 specificity.     

The HLA-DRw8 lineage was generated by a deletion in the DR B region followed by first domain diversification

Sequences were generated for the first, second, and 3'UT regions of DRw8 beta-chain genes from two cell lines differing in their T cell determined allospecificities. Both have second domain sequences homologous to the B1 locus of the DRw52 family (DR3, DR5, and DRw6) and not the B3 locus. However, the 3'UT sequence is homologous to the 3'UT region of the B3 locus of the DRw52 family, and not the B1 locus. The first domain sequences are B1-like as opposed to B3-like and show polymorphism in the region encoding the putative alpha-helical region of the DR beta-chain. The easiest interpretation is that the DRw8 haplotypes constitute a sublineage within the DRw52 group and that this lineage has arisen by a small chromosomal deletion of the region between the B1 locus and the B3 locus. This deletion included the 3'UT region of the B1 locus, the B2 pseudogene, and the 5' end of the B3 locus including the exons encoding the first and second domains. After the deletion, two changes in the first domain arose by a mutational mechanism, possibly gene conversion. One of these changes parallels one seen in the DRw11 lineage.     

Structural analysis of anti-DR1 allorecognition by using DR1/H-2Ek hybrid molecules. Influence of the beta 2-domain correlates with CD4 dependence.

A segmental analysis of the key regions of HLA-DR1 that control T cell allorecognition was performed by using a series of transfected cell lines expressing the products of recombinant DRB/H-2Eb genes, paired with either DR alpha or H-2E alpha. Four of eight human T cell clones tolerated substitution of the H-2E alpha chain, but only one clone showed any response to the DR alpha/H-2E beta k dimer. Both the membrane-proximal and the membrane-distal domains of the beta-chain played an important part in stimulating these clones. The response of four of eight clones was markedly inhibited by substitution of the H-2E beta 2 for the DR beta 2 domain. This inhibition showed a complete correlation with the sensitivity of the clones to inhibition by anti-CD4 mAb. Taken together, these results suggest that the interaction site for CD4 may include residues on the beta 2-domain. Introduction of H-2Ek sequence into either half of the beta 1-domain led to a complete loss of response by all but two of the clones. This is consistent with these clones having dual specificity for exposed DR1-specific polymorphisms and for DR1-bound peptides. The pattern of response of one of the clones suggested that indirect conformational effects on the alpha 1-domain may also contribute to the influence of the amino-terminal half of the beta 1-domain on T cell recognition. In the presence of H-2E alpha, this clone responded more strongly when the amino-terminal half of the beta 1-domain was of H-2Ek rather than DR1 sequence. This implies that species matching of the floor of the beta 1-domain with the alpha-chain is more important than the presence of the alpha-chain of the parental species.     

Beta-chain broadens range of CD8 recognition for MHC class I molecule.

It is known that the alpha-chain of CD8 binds to a negatively charged loop composed of residues 223 to 229 on MHC class I Ag and that binding of CD8 alpha enhances Ag recognition of T cells. We have recently shown that the mouse CD8 alpha homodimer does not bind to either the HLA class I alpha 3 domain or a mutant of H-2Kb Ag containing a substitution of glutamine for methionine at residue 224, which brings this residue toward the human consensus. Here we report a complementary study of the CD8 beta-chain. The functional role of the CD8 beta-chain was analyzed by using four T cell hybridoma lines expressing mouse CD8 alpha and transfected with the mouse CD8 beta gene. As compared with the lines expressing only CD8 alpha, allorecognition of the chimeric H-2Kb Ag that contains the HLA class I alpha 3 domain was enhanced in lines expressing both CD8 alpha and -beta. This enhancement was blocked by either anti-CD8 mAb or anti-HLA class I alpha 3 domain mAb. In addition, we show that CD8 alpha beta binds the H-2Kb mutant Ag at residue 224. These results suggest that the beta-chain allows the CD8 alpha beta heterodimer to recognize the chimeric H-2Kb Ag. A model for the role of the beta-chain is presented.     

Complete characterization and sequence of an HLA class II DR beta chain cDNA from the DR5 haplotype

The human major histocompatibility complex includes the DP, DQ, and DR subregions, each of which contains at least one alpha chain gene and two beta chain genes. The products of the alpha chain gene and a beta chain gene from a given subregion combine to form a heterodimer which is found predominantly on the surface of immunocompetent cells, and is essential for effective cell-cell interactions and the generation of an immune response. The beta chain of the DR molecule is highly polymorphic, and it is this polymorphism which is thought to be ultimately responsible for the specific immune responsiveness and disease predisposition conferred by different DR molecules. While the sequences of DR beta chains of the homozygous DR1 cells, homozygous DR2, homozygous DR4, DR3/w6 cells and DR4/w6 genotypes have been partially or completely characterized, no sequence is yet available for the DR beta chain from a homozygous DR5 cell. A cDNA library was therefore constructed from the Swei cell line homozygous for the DR5 haplotype. A beta chain clone was isolated, characterized, and sequenced. Comparison with previously published DR beta chain restriction endonuclease maps and nucleotide sequences demonstrated that this clone was a DR beta chain clone. Comparison of the deduced amino acid sequence with other DR beta chain amino acid sequences shows three regions of variability in the first external domain, corresponding to amino acid residues 9-13, 26-38, and 67-74. The sequence of each of these variable regions in the beta chain from DR5 cells was identical or nearly identical to the sequences of variable regions found in the beta chains of other DR haplotypes, supporting the notion of gene conversion as an evolutionary mechanism generating polymorphism. The second external domain, and transmembrane and intracytoplasmic regions show a high degree of sequence conservation.     

Complexity of the supertypic HLA-DRw53 specificity: two distinct epitopes differentially expressed on one or all of the DR beta-chains depending on the HLA-DR allotype

The supertypic HLA-DRw53 specificity is associated with three allelic class II specificities defined by alloantisera: HLA-DR4, -DR7, and DRw9. The present study demonstrates the complexity of this supertypic DR specificity by comparing two DRw53-related determinants defined by the monoclonal antibodies PL3 and 109d6. For every HLA-DR4 cell line tested, both monoclonal antibodies were found to bind to the same subpopulation of DR molecules. This PL3+, 109d6+ DR subpopulation is also found on most, but not all, DR7+ cell lines with a beta-chain pattern that is identical to the beta-chain pattern of the PL3+, 109d6+ subpopulation on DR4 cell lines. However, some DR7+ cells which carry the HLA haplotype Bw57, DR7, DRw53, DQw3 were also found which completely lack the expression of the 109d6 determinant, but continue to express the PL3 determinant and some of the DRw53 determinants recognized by alloantisera. This results from the fact that the PL3 determinant is expressed on all of the DR molecules found on DR7 cells, including the distinct subpopulation of molecules that carry the HLA-DR7 determinant recognized by the monoclonal antibody SFR16-DR7. This PL3+, SFR16-DR7+ subpopulation does not carry the 109d6 determinant, demonstrating that the PL3 and 109d6 DRw53-related determinants are distinct and can be expressed on a different number of DR molecules, depending on the allotype of the cells. Blocking studies were also performed by using these monoclonal antibodies with alloreactive HLA-DR7-specific cytotoxic T cell clones. In these studies, the T cell-defined HLA-DR7 determinants were found to be carried by the same subpopulation of DR molecules recognized by the HLA-DR7-specific monoclonal antibody and not carried by the DR molecules recognized by 109d6. The DR7+ cell lines which do not express the 109d6 determinant also fail to express another supertypic determinant recognized by the monoclonal antibody IIIE3 carried on this molecule. Furthermore, no additional allelic forms of this unique DR beta-chain were found associated with the nonpolymorphic DR alpha-chain on these cells, suggesting that this DR beta-chain gene is not expressed. These cells also behave as homozygous typing cells for the Dw11 subtype of DR7 in HLA-D typing in the mixed lymphocyte culture assay. This suggests that the lack of expression of a specific class II gene may contribute additional genetic polymorphism within the known HLA-DR allotypes.     

Inhibition of HIV infection by a novel CD4 domain 2-specific monoclonal antibody. Dissecting the basis for its inhibitory effect on HIV-induced cell fusion.

HIV use the CD4 molecule as their primary cellular receptor. Residues in the N-terminal domain (D1) of CD4 are crucial to HIV attachment through the gp120 envelope component. However, other regions of CD4 appear to be required subsequently for virus- and cell-cell fusion. Little is understood of the post-binding steps which may differ between HIV variants. We report a novel anti-CD4 mAb that does not block CD4/gp120 binding, but that does efficiently block both viral infection and cell-cell syncytia formation, and define its contact site as residues in CD4 D2 using both mouse/human CD4 chimeras and CD4 substitution mutants. We also investigated the basis for its antiviral effect. Using the CD4 D2 specific mAb, we identify another conserved step in HIV infection, as evidenced by its ability to neutralize a broad range of primary isolates and T cell-line passaged strains. Monovalent forms of the mAb were used to determine if its activity was due to masking of the D2 epitope, to steric inhibition, or bivalency. Our data indicate that both binding site and bivalency of the mAb underlie its potency. The need for bivalency is not simply explained by affinity, because monovalent forms can displace the intact mAb and reverse its protective effect. These results provide evidence that binding of the D2-specific mAb prevents structural alterations necessary for membrane fusion.     

Epitope mapping of an HLA-DR-specific monoclonal antibody produced by using human-mouse transfectant cells

A polymorphic HLA-DR mAb, TAL15.1, was produced against L cells transfected with DR alpha- and beta-chain cDNA from a cell line homozygous for HLA-DRw8. This antibody reacted with DRw8 plus all other DR types except DR3 and DRw52. DR3 and DRw52 differ uniquely from other other DR antigens at position 77 in the beta 1-domain of their beta-chains where there is asparagine instead of threonine. In Western blots the antibody reacted with DR alpha/beta-dimer but not with free alpha- or beta-chains. Two-dimensional gel analysis of a DRw11, DRw52 cell line showed that TAL15.1 immunoprecipitated the DR products. Although it also coprecipitated the DQ beta chain products, flow microfluorimetric analysis with various transfectant cell lines showed that TAL15.1 failed to bind the DQ or DP products tested. We conclude that TAL15.1 is a DR-specific polymorphic antibody whose activity correlates with a specific residue. It has already proved to be a valuable reagent for distinguishing DR3 homozygotes from DR3, DRw6 heterozygotes, which have in the past been difficult to separate.     

Polymorphic HLA-DR7 beta 1 chain residues that are involved in T cell allorecognition

The contributions to allorecognition of polymorphic amino acids in the HLA-DR7 beta 1 chain were analyzed by using mutant DR7 beta 1 chains with single amino acid substitutions at position 4, 11, 13, 25, 30, 37, 57, 60, 67, 70, 71, 74, or 78. Transfectants expressing mutant DR7 molecules were used as stimulators for six DR7-alloreactive T cell clones. The majority of the substitutions had profound effects on the ability of the DR7 molecule to stimulate one or more T cell clones. Nine of the 13 substitutions completely abrogated recognition by at least one clone. The finding that each of the substitutions in the beta-strands in the floor of the peptide binding groove affected T cell allorecognition supports the model of allorecognition in which the complex of a self-peptide bound to a class II molecule is recognized by the TCR. Interestingly, the substitution at position 4, which is predicted to be located outside the peptide binding groove, decreased the ability of the DR7 molecule to stimulate some clones. Each of the DR7-alloreactive T cell clones had a unique reactivity pattern in response to the different mutant molecules, indicating that the TCR of each clone recognized the DR7 molecule differently. Surprisingly, many of the mutant DR7 molecules induced proliferation by one or more clones that was greater than 125% of the proliferation induced by the wild-type DR7 molecule. These data indicate that multiple polymorphic residues, predicted in the class II model to be located in both the beta-strands and alpha-helix of the DR7 beta 1 chain, contribute to allorecognition of the DR7 molecule.     

Three-Dimensional Analysis of CD6 Site-Directed Mutagenesis and Monoclonal Antibody Binding Studies Using the X-ray Structure of Mac-2 Binding Protein and a Molecular Model of the CD6 Ligand Binding Domain

The extracellular region of CD6 consists of three scavenger receptor cysteine-rich (SRCR) domains and binds activated leukocyte cell adhesion molecule (ALCAM), a member of the immunoglobulin superfamily (IgSF). Residues important for the CD6-ALCAM interaction have previously been identified by mutagenesis. A total of 22 CD6 residues were classified according to their importance for anti-CD6 monoclonal antibody (mAb) and/or ALCAM binding. The three-dimensional structure of the SRCR domain of Mac-2 binding protein has recently been determined, providing a structural prototype for the SRCR protein superfamily. This has made a thorough three-dimensional analysis of CD6 mutagenesis and mAb binding experiments possible. Mutation of buried residues compromised both mAb and ALCAM binding, consistent with the presence of structural perturbations. However, several residues whose mutation affected both mAb and ALCAM binding or, alternatively, only ligand binding were found to map to the surface in the same region of the domain. This suggests that the CD6 ligand binding site and epitopes of tested mAbs overlap and provides an explanation for the finding that these mAbs effectively block ALCAM binding. An approximate molecular model of CD6 was used to delineate the ALCAM binding site.Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1007/s0089490050263Abbreviations ALCAM activated leukocyte cell adhesion molecule - CD6D3 third (membrane-proxi-mal) extracellular domain of CD6 - IgSF immunoglobulin superfamily - mAb monoclonal antibody - M2BP Mac-2 binding protein - SRCR scavenger receptor cysteine-rich domain - SRCRSF scavenger receptor cysteine-rich protein superfamily     

Molecular localization to a unique DR beta-chain of the restriction element of an anti-influenza T cell clone

The DR beta-chains of a panel of DRw13 cells were characterized by two-dimensional polyacrylamide gel electrophoresis in order to identify the molecule bearing the restriction element of a DR-restricted proliferative and cytotoxic T cell clone COT C2 specific for DRw13 and the influenza A virus. Because in different DRw13 haplotypes one DRw13 beta-chain can be associated with other distinct DRw13 beta-chains, such complex associations allowed us to determine the ability of a given DR beta-chain to present the antigen or COT C2. The presence of the DR beta-chain 6B5, and only this chain, is associated with the ability to induce the proliferation of clone COT C2 whatever the second DR beta-chain associated with 6B5 is, namely 6B4 or 6B3. The DRw13 cells that express 6B4 or 6B3 but not 6B5 could not present the antigen to COT C2. Moreover, ILR2, a monoclonal antibody that precipitated 6B5, blocked the proliferation of COT C2, whereas 7.3.19.1, a monoclonal antibody that precipitated only 6B4 and 6B3, could not block the proliferation of this clone. Thus, the DRw13 beta-chain 6B5 appears as the unique class II element that restricts the response of the T cell clone COT C2.     

Analysis of the molecular specificities of anti-class II monoclonal antibodies by using L cell transfectants expressing HLA class II molecules

Expressible HLA class II alpha- and beta-chain cDNA were used for DNA-mediated gene transfer to produce L cell transfectants expressing single types of human class II molecules. Cloned transfectants expressing nine different class II molecules were isolated: DR alpha: DR1 beta I, DR alpha: DR4 beta I, DR alpha: DR5 beta I, DR alpha: DR5 beta III (DRw52), DR alpha: DR7 beta I, DR alpha: DR4/7 beta IV (DRw53), DQ7 alpha: DQw2 beta, DQ7 alpha: DQw3 beta, and DPw4 alpha: DPw4 beta. These class II-expressing transfectants were used to analyze by flow cytometry the molecular specificities of 20 anti-class II mAb. These analyes indicate that some mAb are more broadly reactive than was previously thought based on immunochemical studies. In contrast, the narrow molecular specificities of other anti-class II mAb were confirmed by this approach. Transfectants expressing human class II molecules should be valuable reagents for studies of B cell and T cell defined epitopes on these molecules.     

Antigen-specific T cells with monogamous or promiscuous restriction patterns are sensitive to different HLA-DR beta chain substitutions

The contributions of the amino acids at 13 polymorphic positions in the HLA-DR7 beta 1 chain to T cell recognition of two antigenic peptides of tetanus toxin (p2 and p30) were assessed using transfectants expressing mutant DR7 beta 1 chains as APC for six toxin-specific T cell clones with two different restriction patterns: monogamous (restricted by DR7 only) or promiscuous (restricted by DR7; DR1; DR2, Dw21; and DR4, Dw4). Each of the 13 substitutions significantly decreased or eliminated the ability of the DR7 molecule to present a peptide to one or more of the T cell clones, but none of the substitutions abolished recognition by all clones. Interestingly, substitutions at positions 4 and 25, which are predicted in the class II model to be located outside the peptide binding groove, decreased the ability of the DR7 molecule to present Ag to some clones but not to others. Each of the four clones specific for the p2 peptide and the two clones specific for peptide p30 had a different reactivity pattern to the panel of DR7 beta 1 mutants, indicating that the TCR of each clone has a different view of the p2/DR7 or p30/DR7 complex. These data emphasize the complexity of the interactions of multiple residues in DR7 beta 1 chains in Ag-specific T cell recognition.     

Species specificity in the interaction of CD8 with the alpha 3 domain of MHC class I molecules.

The alpha 1 and alpha 2 domains of the class I MHC molecule constitute the putative binding site for processed peptides and the TCR, although the alpha 3 domain has been implicated as a binding site for the CD8 molecule. Species specificity in the binding of CD8 to the alpha 3 domain has been suggested as an explanation for the low xenogeneic T cell response to class I molecules, but results on this point have been conflicting and controversial. We have addressed this issue using CTL lines from HLA-A2.1 transgenic mice that specifically recognize and lyse A2.1-expressing cells infected with influenza A/PR/8 or pulsed with influenza matrix peptide M1(57-68). Species specificity was examined using transfectants that expressed hybrid molecules containing the alpha 1 and alpha 2 domains from HLA-A2.1 and the alpha 3 domain from a murine class I molecule. Lower levels of M1(57-68) peptide were required to sensitize L cell transfectants expressing a chimera that contained an H-2Dd alpha 3 domain than targets expressing the intact A2.1 molecule. However, at high doses of peptide, lysis of these two targets was similar. However, no reproducible difference in sensitization was observed using EL4 or Jurkat transfectants expressing A2.1 or A2.1 chimeric molecules that contained an H-2Kb alpha 3 domain. In all cases, however, lysis of peptide-pulsed A2.1 expressing targets was more sensitive to inhibition with anti-CD8 mAb than lysis of cells expressing these chimeric molecules. Thus, under suboptimal conditions such as low Ag density or in the presence of anti-CD8 mAb, these CTL preferentially recognize class I molecules with a murine alpha 3 domain. This suggests that there is some species specificity in the interaction of CD8 with the alpha 3 domain of the class I molecule. However, CTL recognition was inhibited by point mutations in the alpha 3 domain of HLA-A2.1 that have been shown to inhibit binding of human CD8 and recognition by human CTL, suggesting that murine CD8 interacts to some degree with human alpha 3 domains, and that similar alpha 3 domain residues may be important for murine and human CD8 binding. The relevance of these results to an understanding of low xenogeneic responses is discussed.