Invariant chain association with MHC class I: preference for HLA class I/beta 2-microglobulin heterodimers, specificity, and influence of the MHC peptide-binding groove

MHC class I molecules require the assembly of heavy chain with beta2-microglobulin (beta 2m) and peptide in order to present Ag on the cell surface. Endoplasmic reticulum resident proteins associate with class I molecules and aid assembly. Free class I heavy chains associate with calnexin, which may facilitate association with beta 2m. Invariant chain (Ii) also associates with MHC class I molecules, but its role in class I assembly is not clear. We report here that Ii strongly associates with HLA class I/beta 2m heterodimers, but weakly with free class I heavy chains in HLA-B7-transfected T2 cells. Ii/HLA class I complexes persist stably within the endoplasmic reticulum/cis-Golgi compartment in peptide-processing deficient cells, but are much less prominent in normally processing cells. Furthermore, Ii differentially associates with variant HLA-B7 molecules that have peptide-binding groove mutations, and the degree of association correlates with HLA-B7 variant cell surface expression. Ii also shows HLA class I molecule specificity, associating to a greater degree with HLA-B7 than HLA-A2. Together these observations suggest that Ii stabilizes particular HLA class I/beta 2m heterodimers until peptide is loaded, and that this association may enhance class I cell surface expression.

Invariant chain association with MHC class I: preference for HLA class I/beta 2-microglobulin heterodimers, specificity, and influence of the MHC peptide-binding groove

MHC class I molecules require the assembly of heavy chain with beta2-microglobulin (beta 2m) and peptide in order to present Ag on the cell surface. Endoplasmic reticulum resident proteins associate with class I molecules and aid assembly. Free class I heavy chains associate with calnexin, which may facilitate association with beta 2m. Invariant chain (Ii) also associates with MHC class I molecules, but its role in class I assembly is not clear. We report here that Ii strongly associates with HLA class I/beta 2m heterodimers, but weakly with free class I heavy chains in HLA-B7-transfected T2 cells. Ii/HLA class I complexes persist stably within the endoplasmic reticulum/cis-Golgi compartment in peptide-processing deficient cells, but are much less prominent in normally processing cells. Furthermore, Ii differentially associates with variant HLA-B7 molecules that have peptide-binding groove mutations, and the degree of association correlates with HLA-B7 variant cell surface expression. Ii also shows HLA class I molecule specificity, associating to a greater degree with HLA-B7 than HLA-A2. Together these observations suggest that Ii stabilizes particular HLA class I/beta 2m heterodimers until peptide is loaded, and that this association may enhance class I cell surface expression.

Alloreactive anti-HLA-B7 cytolytic T cell clones use restricted T cell receptor genes

Most alloreactive T cells specifically recognize peptides bound to donor MHC molecules. In addition to peptides, solvent accessible MHC residues also may stimulate alloreactive T cells. We studied T cell receptor (TCR) usage by 16 independent anti-HLA-B7 alloreactive cytolytic T lymphocyte (CTL) clones. Most or all of these CTL clones recognized unique peptides bound to HLA-B7. Despite the diversity of peptides recognized, 11 out of 15 CTL clones analyzed expressed TCR V(alpha) gene segment (AV) subgroups 1 and 3. Within AV subgroup 1, four of six clones expressed AV2; within AV subgroup 3, three clones used AV6. Ten of 14 CTL clones analyzed expressed V(beta) gene segment (BV) subgroups 4 and 1. In subgroup 4, BV14 was expressed by four of five alloreactive CTL clones. Similar AV and BV usage restriction was not found in mitogen-stimulated peripheral blood T cells from the major donor of the CTL clones. TCR A and TCR B junctional region sequences were quite diverse in length and sequence, although two CTL clones expressed nearly identical TCR B chains. We found no correlation between TCR AV or TCR BV usage and CTL recognition of 81 HLA-B7 variants. These results are consistent with models of TCR structure, in which very diverse TCR CDR3 regions contact very diverse peptides, and moderately diverse TCR CDR1 and CDR2 regions contact moderately diverse MHC alpha-helices.

The resolution of the neutral N-linked oligosaccharides of IgG by high pH anion-exchange chromatography

The introduction of high pH anion-exchange chromatography (HPAEC) has represented a major development in the qualitative analysis of glycoprotein derived oligosaccharides. When coupled with pulsed amperometric detection, the technique permits the detection of picomole quantities of heterogeneous mixtures of oligosaccharide without the need for derivatisation. The applications of HPAEC have generally been limited to the analysis of sialylated oligosaccharides, however, it is now possible to analyse heterogenous mixtures of neutral oligosaccharides with the latest systems. We have used such a system to separate completely a panel of seven commercially available neutral N-linked oligosaccharides and found the influence of monosaccharide substitution on elution position to be identical to that for sialylated structures. A standard monosialylated N-linked oligosaccharide was modified by sequential digestion with specific exoglycosidases to produce a monogalactosylated, diantennary oligosaccharide which is commercially unavailable. This standard's elution position was confirmed by HPAEC. The technique was applied to the identification of neutral N-linked oligosaccharides released from human immunoglobulin G using the enzyme peptide-N-glycosidase F.

Probing HLA-B7 conformational shifts induced by peptide-binding groove mutations and bound peptide with anti-HLA monoclonal antibodies

To determine the influence of peptide-binding groove residues and MHC-bound peptide on HLA-B7 conformation, we investigated the binding sites of nine locus- or allele-specific mAbs using a panel of 82 HLA-B7 variants. The functional mAb epitopes encircle the HLA-B7 peptide-binding groove. Three mAbs are affected by mutations at solvent-accessible peptide-binding groove mutations. Mutations in peptide-binding groove residues 45, 63, and 150 affect multiple nonoverlapping mAb epitopes, probably by interaction with other MHC residues or bound peptide. However, 18 of 24 peptide-binding groove mutations do not affect mAb binding, indicating that the conformation of solvent-accessible HLA-B7 structures is largely dissociated from changes in the peptide-binding groove. To test whether bound peptides alter HLA-B7 conformation, we loaded HLA-B7 heavy chains on acid-stripped cells with beta2-microglobulin and 20 individual synthetic peptides. Two of eight mAbs are sensitive to HLA-B7-bound peptides. A likely interpretation of these data is that the conformational flexibility of HLA-B7 is due to peptide-induced conformational shifts in MHC side chains, rather than major shifts in the MHC main chain. These results suggest that HLA-B7 conformation is largely maintained in the context of different bound peptides and different peptide-binding grooves.

Probing HLA-B7 conformational shifts induced by peptide-binding groove mutations and bound peptide with anti-HLA monoclonal antibodies

To determine the influence of peptide-binding groove residues and MHC-bound peptide on HLA-B7 conformation, we investigated the binding sites of nine locus- or allele-specific mAbs using a panel of 82 HLA-B7 variants. The functional mAb epitopes encircle the HLA-B7 peptide-binding groove. Three mAbs are affected by mutations at solvent-accessible peptide-binding groove mutations. Mutations in peptide-binding groove residues 45, 63, and 150 affect multiple nonoverlapping mAb epitopes, probably by interaction with other MHC residues or bound peptide. However, 18 of 24 peptide-binding groove mutations do not affect mAb binding, indicating that the conformation of solvent-accessible HLA-B7 structures is largely dissociated from changes in the peptide-binding groove. To test whether bound peptides alter HLA-B7 conformation, we loaded HLA-B7 heavy chains on acid-stripped cells with beta2-microglobulin and 20 individual synthetic peptides. Two of eight mAbs are sensitive to HLA-B7-bound peptides. A likely interpretation of these data is that the conformational flexibility of HLA-B7 is due to peptide-induced conformational shifts in MHC side chains, rather than major shifts in the MHC main chain. These results suggest that HLA-B7 conformation is largely maintained in the context of different bound peptides and different peptide-binding grooves.

Peptide-dependent expression of HLA-B7 on antigen processing-deficient T2 cells

Class I MHC Ag presentation and cell surface expression largely depend on peptide transport into the ER/cis-Golgi by TAP, the transporter associated with Ag processing. Despite this dependency, class I MHC molecules are expressed at low levels on the surface of TAP-deficient T2 cells. We studied the peptide dependency of HLA-B7 expression in transfected T2 cells. HLA-B7 expression was affected by mutations at 19 out of 23 peptide-binding groove residues, but not by nine mutations outside of the peptide-binding groove. T2 cell surface HLA-A2, -B7, and -B51 had similar stabilities, and approximately half of these class I molecules had a long t1/2 consistent with tight peptide binding. Using metabolically labeled T2 cells, HLA-A2-bound peptide eluted as five prominent peaks, but HLA-B7-bound peptide was not detected. In contrast, HLA-B7-eluted peptides were detected spectrophotometrically. These data suggest that HLA-A2 and HLA-B7 molecules utilize distinct TAP-independent peptide supply mechanisms to different degrees. Equivalent amounts of HLA-B7 from TAP- and TAP+ cells yielded similar amounts of peptide, which had the characteristic HLA-B7 peptide-binding motif. The dependency of HLA-B7 cell surface expression on peptide-binding groove residues, the stability of cell surface class I molecules, and the ability to detect HLA-B7-bound peptide indicate that the low level expression on T2 cells is largely peptide dependent. TAP-independent peptide Ag presentation may allow immune recognition of intracellular pathogens that interfere with TAP-dependent peptide transport.

Peptide-dependent expression of HLA-B7 on antigen processing-deficient T2 cells

Class I MHC Ag presentation and cell surface expression largely depend on peptide transport into the ER/cis-Golgi by TAP, the transporter associated with Ag processing. Despite this dependency, class I MHC molecules are expressed at low levels on the surface of TAP-deficient T2 cells. We studied the peptide dependency of HLA-B7 expression in transfected T2 cells. HLA-B7 expression was affected by mutations at 19 out of 23 peptide-binding groove residues, but not by nine mutations outside of the peptide-binding groove. T2 cell surface HLA-A2, -B7, and -B51 had similar stabilities, and approximately half of these class I molecules had a long t1/2 consistent with tight peptide binding. Using metabolically labeled T2 cells, HLA-A2-bound peptide eluted as five prominent peaks, but HLA-B7-bound peptide was not detected. In contrast, HLA-B7-eluted peptides were detected spectrophotometrically. These data suggest that HLA-A2 and HLA-B7 molecules utilize distinct TAP-independent peptide supply mechanisms to different degrees. Equivalent amounts of HLA-B7 from TAP- and TAP+ cells yielded similar amounts of peptide, which had the characteristic HLA-B7 peptide-binding motif. The dependency of HLA-B7 cell surface expression on peptide-binding groove residues, the stability of cell surface class I molecules, and the ability to detect HLA-B7-bound peptide indicate that the low level expression on T2 cells is largely peptide dependent. TAP-independent peptide Ag presentation may allow immune recognition of intracellular pathogens that interfere with TAP-dependent peptide transport.

Analysis of the glycosylation patterns of the extracellular domain of the epidermal growth factor receptor expressed in Chinese hamster ovary fibroblasts

The extracellular domain (621 N-terminal amino acids) of the p170 epidermal growth factor (EGF) receptor has eleven consensus N-linked glycosylation sites. When expressed in Chinese hamster ovary cells this was glycosylated with a combination of high mannose and complex chains. The latter chains were shown by chromatographic separation and mass spectrometric analysis of tryptic digests to be clustered in the EGF-binding domain. Treatment with the endoglycosidase, peptide-N-glycosidase F (PNGase F), reduced the molecular weight from 110 kDa to 75 kDa. Released oligosaccharides were characterised at high sensitivity by high pH anion exchange chromatography with pulsed amperometric detection and gas-liquid chromatography/mass spectrometry. The data were consistent with the complex chains being trisialylated tetra-antennary oligosaccharides fucosylated on the reducing terminal GlcNAc. The large hydrodynamic mass of these oligosaccharides could influence ligand binding, an effect which is likely to vary with the difference in consensus glycosylation sites of proteins related to p170 i.e. p185erbB2/neu, p180erbB3 and p180erbB4.

Chromatographic investigation of the glycosylation pattern of alpha-1-acid glycoprotein secreted by the HepG2 cell line; a putative model for inflammation?

In certain pathophysiological conditions, such as rheumatoid arthritis, there are alterations in the glycosylation pattern of the acute phase protein, alpha-1-acid glycoprotein (AGP). These changes are likely to be functionally significant, however, verification of the latter role requires a system which reflects in vivo glycosylation changes in AGP and also produces sufficient quantities of the protein for further study. The human hepatoma cell line HepG2 is documented as displaying a shift in the glycosylation pattern of glycoproteins from normal state to acute phase after stimulation with inflammatory mediators. We have isolated AGP from the culture medium of HepG2 cells both before and after stimulation with a cytokine preparation and analysed the glycosylation pattern of each preparation, after enzymatic release, by high pH anion-exchange chromatography. Before stimulation, the glycosylated population was similar to a profile of AGP isolated from normal plasma; however, cytokine stimulation resulted in a shift to a profile which was consistent with that of AGP from a rheumatoid arthritis sufferer. Thus a HepG2 cell culture system is capable of being a crude model of the changes in glycosylation of acute phase proteins although it has a tendency to produce oligosaccharide chains which are not fully sialylated.

Tumor necrosis factor-alpha and X-linked adrenoleukodystrophy

The two most common forms of X-linked adrenoleukodystrophy (X-ALD), the childhood cerebral form (CCER) and the adult form, adrenomyeloneuropathy (AMN), arise from the same mutations in the X-ALD gene at Xq28. These two forms are distinguished by the degree of cerebral inflammation. Segregation analysis suggests that an autosomal modifying gene may be a major determinant of phenotype in X-ALD. Thus, a modifying gene could be involved in initiating or promoting the inflammatory response. In this study we detected a difference in tumor necrosis factor-alpha (TNF-alpha) bioactivity, but not TNF-alpha protein levels, in serum from some advanced CCER patients. Early-stage CCER patients and AMN patients were in the normal range. Allelic differences in TNF-alpha or levels of soluble TNF receptor did not account for bioactivity differences or phenotypic heterogeneity in X-ALD.

Altered expression of ALDP in X-linked adrenoleukodystrophy

X-linked adrenoleukodystrophy (ALD) is a neurodegenerative disorder with variable phenotypic expression that is characterized by elevated plasma and tissue levels of very long-chain fatty acids. However, the product of the gene defective in ALD (ALDP) is a membrane transporter of the ATP-binding cassette family of proteins and is not related to enzymes known to activate or oxidize fatty acids. We generated an antibody that specifically recognizes the C-terminal 18 amino acids of ALDP and can detect ALDP by indirect immunofluorescence. To better understand the mechanism by which mutations in ALDP lead to disease, we used this antibody to examine the subcellular distribution and relative abundance of ALDP in skin fibroblasts from normal individuals and ALD patients. Punctate immunoreactive material typical of fibroblast peroxisomes was observed in cells from seven normal controls and eight non-ALD patients. Of 35 ALD patients tested, 17 had the childhood-onset cerebral form of the disease, 13 had the milder adult phenotype adrenomyeloneuropathy, 3 had adrenal insufficiency only, and 2 were affected fetuses. More than two-thirds (69%) of all patients studied showed no punctate immunoreactive material. There was no correlation between the immunofluorescence pattern and clinical phenotype. We determined the mutation in the ALD gene in 15 of these patients. Patients with either a deletion or frameshift mutation lacked ALDP immunoreactivity, as expected. Four of 11 patients with missense mutations were also immunonegative, indicating that these mutations affected the stability or localization of ALDP. In the seven immunopositive patients with missense mutations, correlation of the location and nature of the amino acid substitution may provide new insights into the function of this peroxisomal membrane protein. Furthermore, the study of female relatives of immunonegative ALD probands may aid in the assessment of heterozygote status.

Adrenoleukodystrophy: molecular genetics, pathology, and Lorenzo's oil

Knowledge about adrenoleukodystrophy (ALD), a disorder which was described first in 1923, has increased greatly during recent years. The principal biochemical abnormality, the presumed enzyme defect, and the gene defect, have been defined. A dietary therapy has been proposed and attracted world-wide attention through a motion picture. Nevertheless, many questions remain and cannot be answered without a more fundamental understanding of pathology and pathogenesis. This article will provide a review of the history, clinical features, pathology, biochemistry, and the gene defect, and then appraise current efforts to clarify pathogenesis and develop therapeutic approaches.

Low HLA-C expression at cell surfaces correlates with increased turnover of heavy chain mRNA

In comparison with HLA-A and -B, the protein products of the HLA-C locus are poorly characterized, in part because of their low level of expression at the cell surface. Here, we examine how protein-protein interactions during assembly and regulation of the mRNA level affect cell surface expression of HLA-C. We find that intrinsic properties of the HLA-C heavy chain proteins do not correlate with low cell surface expression: HLA-C heavy chains associate and dissociate with beta 2-microglobulin (beta 2m) at rates comparable to those found for HLA-A and -B, and increased competition for beta 2m does not alter the surface expression of HLA-C. From studies of chimeric genes spliced from the HLA-B7 and -Cw3 genes, we find that chimeric proteins containing the B7 peptide-binding groove can have low cell surface expression, suggesting that inefficiency in binding peptides is not the cause of low cell surface expression for HLA-C. The surface levels of HLA-A, -B, or -C in cells transfected with cDNA can be similar, implicating noncoding regions of HLA-C heavy chain genes in the regulation of surface expression. We find that HLA-C mRNA is expressed at lower levels than HLA-B mRNA and that this difference results from faster degradation of the HLA-C message. Experiments examining chimeric B7/Cw3 and B7/Cw6 genes suggest that a region determining low expression of HLA-C is to be found between the 3' end of exon 3 and a site in the 3' untranslated region, approximately 600 bases downstream of the translation stop codon.

NK cell recognition of MHC class I. NK cells are sensitive to peptide-binding groove and surface alpha-helical mutations that affect T cells

NK cells directly or indirectly recognize MHC class I molecules, but the exact structures recognized remain poorly defined. We address the hypothesis that NK cells, like T cells, directly contact peptide/MHC class I complexes. This hypothesis predicts that NK-mediated killing is inhibited by amino acid substitutions in the MHC class I peptide-binding groove and in solvent-accessible alpha-helical residues proposed to contact the TCR. In our model system, target cell HLA-B*0702 inhibited killing by unstimulated peripheral blood NK cells. NK-mediated killing was increased significantly by 6 of 11 peptide-binding groove mutations and 6 of 12 TCR contact site mutations, but only 1 of 6 mutations outside these sites. Many of the mutations that inhibited NK-mediated killing prohibited killing by 12 alloreactive CTL clones. These data suggest that NK receptors directly contact HLA-B*0702, focusing on the peptide-binding groove and surrounding alpha-helices. NK cell lines exhibited multiple HLA recognition patterns, which is consistent with nonuniform expression of MHC receptors by NK cells. We propose that NK cells, like alpha beta T cells and some anti-MHC Abs, directly or indirectly recognize MHC-bound peptides.

NK cell recognition of MHC class I. NK cells are sensitive to peptide-binding groove and surface alpha-helical mutations that affect T cells

NK cells directly or indirectly recognize MHC class I molecules, but the exact structures recognized remain poorly defined. We address the hypothesis that NK cells, like T cells, directly contact peptide/MHC class I complexes. This hypothesis predicts that NK-mediated killing is inhibited by amino acid substitutions in the MHC class I peptide-binding groove and in solvent-accessible alpha-helical residues proposed to contact the TCR. In our model system, target cell HLA-B*0702 inhibited killing by unstimulated peripheral blood NK cells. NK-mediated killing was increased significantly by 6 of 11 peptide-binding groove mutations and 6 of 12 TCR contact site mutations, but only 1 of 6 mutations outside these sites. Many of the mutations that inhibited NK-mediated killing prohibited killing by 12 alloreactive CTL clones. These data suggest that NK receptors directly contact HLA-B*0702, focusing on the peptide-binding groove and surrounding alpha-helices. NK cell lines exhibited multiple HLA recognition patterns, which is consistent with nonuniform expression of MHC receptors by NK cells. We propose that NK cells, like alpha beta T cells and some anti-MHC Abs, directly or indirectly recognize MHC-bound peptides.

An analysis of fetal hemoglobin variation in sickle cell disease: the relative contributions of the X-linked factor, beta-globin haplotypes, alpha-globin gene number, gender, and age

Five factors have been shown to influence the 20-fold variation of fetal hemoglobin (Hb F) levels in sickle cell anemia (SS): age, sex, the alpha-globin gene number, beta-globin haplotypes, and an X-linked locus that regulates the production of Hb F-containing erythrocytes (F cells), ie, the F-cell production (FCP) locus. To determine the relative importance of these factors, we studied 257 Jamaican SS subjects from a Cohort group identified by newborn screening and from a Sib Pair study. Linear regression analyses showed that each variable, when analyzed alone, had a significant association with Hb F levels (P < .05). Multiple regression analysis, including all variables, showed that the FCP locus is the strongest predictor, accounting for 40% of Hb F variation. beta-Globin haplotypes, alpha-globin genes, and age accounted for less than 10% of the variation. The association between the beta-globin haplotypes and Hb F levels becomes apparent if the influence of the FCP locus is removed by analyzing only individuals with the same FCP phenotype. Thus, the FCP locus is the most important factor identified to date in determining Hb F levels. The variation within each FCP phenotype is modulated by factors associated with the three common beta-globin haplotypes and other as yet unidentified factor(s).

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.

Heterogeneity of alpha 1-acid glycoprotein in rheumatoid arthritis

alpha 1-Acid glycoprotein (AGP) or orosomucoid is a major serum glycoprotein, of unknown physiological function, which is classified as one of the positive acute phase reactants since its plasma concentration becomes elevated two- to five-fold in certain disease states. Additionally, the proportions and identities of the five asparaginyl-linked complex oligosaccharide chains are altered during several physiological and pathological conditions, which may be functionally significant. The key to studying the structural heterogeneity of AGP is to develop a procedure that will isolate AGP without structural degradation. We have developed a method for the purification of AGP, using procedures unlikely to damage the glycoprotein structure, which was utilised to isolate AGP from samples of normal and rheumatoid plasma. The effectiveness of the purification procedure was examined by enzymatically deglycosylating each sample of AGP and separating the released oligosaccharides by chromatography on a pellicular high-performance anion-exchange (HPAE) resin at pH 13. The analytical profile for normal AGP was consistent with that previously reported thus indicating that the purification procedure did not denature the oligosaccharide chains of AGP. Additionally, there was a noticeable difference between the profiles for AGP from normal and rheumatoid plasma.

Bw4-reactive and Bw6-reactive antibodies recognize multiple distinct HLA structures that partially overlap in the alpha-1 helix

Bw4 and BW6 epitopes are expressed by mutually exclusive sets of HLA-B alleles and some HLA-A and HLA-C alleles. To test whether antithetical structures are required to express Bw4 and Bw6 epitopes, we measured binding of Bw4-reactive and Bw6-reactive alloantibodies and mAbs to HLA-B7 variants. A triple substitution of HLA-B7 alpha-1 helix residues 80, 82, and 83 created Bw4 and destroyed Bw6 epitopes detected by alloantibodies and mAbs. Both Bw4-reactive and Bw6-reactive mAbs competed for binding to HLA-B7 variants with single substitutions at residues 82 and 83. Substitutions of residues H93 and D119 which form a salt bridge in HLA-A2 also permitted binding by both Bw4-reactive and Bw6-reactive mAbs, suggesting that Bw4 and Bw6 epitopes are conformationally dependent. Six Bw4-reactive mAbs showed four distinct patterns of binding to HLA-B7 variants. Detailed analysis of 74 HLA-B7 single-residue variants showed that Bw6-reactive SFR8-B6 binding was prohibited by mutations altering the distal end of the alpha-1 helix and the nearby connecting loop. In contrast, Bw6-reactive BB7.6 binding required both alpha-1 and alpha-2 helix residues. Thus, Bw4-reactive and Bw6-reactive Abs recognize multiple distinct HLA structures that partially overlap in the alpha-1 helix. As both Bw4 and Bw6 epitopes are expressed by some HLA-B7 variants, mutually exclusive expression of Bw4 and Bw6 epitopes in naturally occurring HLA class 1 molecules may reflect evolutionary pressure.