Alloreactive T cell recognition of MHC class I molecules: the T cell receptor interacts with limited regions of the MHC class I long alpha helices

T cells recognize MHC-bound peptide, suggesting that the TCR contacts surface MHC residues adjacent to bound peptide, but the extent of MHC contact is not known. T cells also may recognize peptide-induced conformational changes, and alloreactive T cells may recognize surface MHC structures in addition to or independent of bound peptide. Alloreactive T cells are not intentionally biased to recognize particular MHC-bound peptides and should reveal general constraints for TCR binding. To map TCR binding sites, we tested 60 HLA-B7 site-specific mutations with 12 alloreactive CTL clones that express different TCRs. The alloreactive CTL clones recognize solvent-accessible residues that cluster between positions 62 to 80 and 150 to 170. Thus, TCRs contact largely overlapping MHC structures in the alpha1 and alpha2 domain long alpha helices. The dimensions and location of this site are consistent with recently reported crystallographic studies of two TCR/peptide-MHC class I complexes. In contrast to TCR, Abs recognize multiple discrete epitopes that encircle the peptide binding groove and potentially encompass the entire surface of the MHC molecule. Our data suggest that TCRs dock with a common discrete MHC site and that recent crystallographic models are likely to be generally applicable to T cell recognition of peptide-MHC class I complexes.

The relative importance of the X-linked FCP locus and beta-globin haplotypes in determining haemoglobin F levels: a study of SS patients homozygous for beta S haplotypes

Five factors have been hypothesized to influence the 20-fold variation in fetal haemoglobin (Hb F) levels in sickle cell anaemia (SS): age sex, alpha-globin gene number, beta-globin haplotype, and the X-linked F-cell production locus (FCP) that regulates the production of Hb F containing erythrocytes (F cells). We analysed the association of these factors with Hb F levels in 112 SS patients living in France who are homozygous for the three common African beta-globin haplotypes (Benin, Bantu or Central African Republic and Senegal). We found that: (1) FCP accounts for about 40% of the overall variation in Hb F levels, (2) when the FCP influence is removed, beta-globin haplotype is associated with 14% of the remaining Hb F variation, and (3) the other factors have little influence. Comparison with our previous study of SS individuals in Jamaica leads to the following conclusions: (1) the X-linked FCP locus is a major determinant of Hb F levels in SS disease, (2) factors linked to the beta-globin haplotype have only a small effect on the variation in Hb F levels, in either the homozygous or heterozygous state, and (3)approximately half of the variation in Hb F levels still remains to be explained.

Investigation into the concanavalin A reactivity, fucosylation and oligosaccharide microheterogeneity of alpha 1-acid glycoprotein expressed in the sera of patients with rheumatoid arthritis

alpha 1-Acid glycoprotein (AGP) exists as an heterogeneous population of glycosylated variants (glycoforms) in plasma. The concentration of AGP increases some 2-5 fold in certain pathophysiological states exemplified by the chronic inflammatory disease, rheumatoid arthritis (RA). Moreover, the expressed glycosylation pattern alters in such conditions, indicating functional significance that is likely to be related to the oligosaccharide heterogeneity. We have investigated the heterogeneity of AGP glycosylation using the technique of high pH anion-exchange chromatography (HPAEC). AGP was isolated from the blood of RA sufferers, partially separated by Concanavalin A (Con A) affinity chromatography into bound and non-bound fractions and was enzymatically deglycosylated. Chromatography on the pellicular HPAE resin at pH 13 separated the released oligosaccharides and allowed a comparison of profiles in terms of branching and fucosylation. Results demonstrate an abnormal RA AGP glycosylation, with a tendency towards tri- and tetra-antennary oligosaccharides and enhanced fucosylation, in addition to the possible existence of penta-sialylated RA AGP glycoforms.

Conformational changes in MHC class I molecules. Antibody, T-cell receptor, and NK cell recognition in an HLA-B7 model system

In this article we review the role of MHC conformation, including peptide-induced MHC conformation, in forming antibody (Ab), T-cell receptor (TCR), and natural killer (NK) cell receptor epitopes. Abs recognize conformational major histocompatibility (MHC) epitopes that often are influenced by the identity of MHC-bound peptide. Diverse TCRs recognize a common docking site on peptide/MHC complexes and directly contact peptide. Human NK cell inhibitory receptors (KIR) appear to recognize limited regions of the HLA alpha (1) helix. DX9+ KIR specifically focus on HLA-B residues 82 and 83. However, NK cells recognize much broader regions of HLA class I molecules and are sensitive to bound peptides. Thus, several classes of lymphocyte receptors are peptide-specific. Peptide specificity could be the result of direct contact with the receptor, or to conformational shifts in MHC residues that interact with both receptor and bound peptide.

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.