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PubMed1999

Peptide-bound major histocompatibility complex class I molecules associate with tapasin before dissociation from transporter associated with antigen processing.

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to CD8 T cells. The peptides are generated in the cytosol, then translocated across the membrane of the endoplasmic reticulum by the transporter associated with antigen processing (TAP). TAP is a trimeric complex consisting of TAP1, TAP2, and tapasin (TAP-A) as indicated for human cells by reciprocal coprecipitation with anti-TAP1/2 and anti-tapasin antibodies, respectively. TAP1 and TAP2 are required for the peptide transport. Tapasin is involved in the association of class I with TAP and in the assembly of class I with peptide. The mechanisms of tapasin function are still unknown. Moreover, there has been no evidence for a murine tapasin analogue, which has led to the suggestion that murine MHC class I binds directly to TAP1/2. In this study, we have cloned the mouse analogue of tapasin. The predicted amino acid sequence showed 78% identity to human tapasin with identical consensus sequences of signal peptide, N-linked glycosylation site, transmembrane domain and double lysine motif. However, there was less homology (47%) found at the predicted cytosolic domain, and in addition, mouse tapasin is 14 amino acids longer than the human analogue at the C terminus. This part of the molecule may determine the species specificity for interaction with MHC class I or TAP1/2. Like human tapasin, mouse tapasin binds both to TAP1/2 and MHC class I. In TAP2-mutated RMA-S cells, both TAP1 and MHC class I were coprecipitated by anti-tapasin antiserum indicative of association of tapasin with TAP1 but not TAP2. With crosslinker-modified peptides and purified microsomes, anti-tapasin coprecipitated both peptide-bound MHC class I and TAP1/2. In contrast, anti-calreticulin only coprecipitated peptide-free MHC class I molecules. This difference in association with peptide-loaded class I suggests that tapasin functions later than calreticulin during MHC class I assembly, and controls peptide loading onto MHC class I molecules in the endoplasmic reticulum.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1999

The structural basis for the increased immunogenicity of two HIV-reverse transcriptase peptide variant/class I major histocompatibility complexes.

Designing altered peptide ligands to generate specific immunological reactivity when bound to class I major histocompatibility complexes is important for both therapeutic and prophylactic reasons. We have previously shown that two altered peptides, derived from human immunodeficiency virus (HIV)-reverse transcriptase (RT) residues 309-317, are more immunogenic in vitro than the wild-type peptide. One peptide variant, I1Y, was able to stimulate RT-specific cytotoxic T cells from the blood of three HIV-infected individuals better than the wild-type RT peptide. Both I1Y and I1F peptide variants increase the cell surface half-life of the peptide-class I complex approximately 3-fold over that of the RT peptide but have different immunological activities. These peptides are candidates for the design of vaccines for HIV due to their increased immunogenicity. To understand the basis for the increased cell surface stability compared with wild-type peptide and to understand the differences in T cell recognition between I1Y and I1F, we determined the x-ray crystal structures of the two class I MHC-peptide complexes. These structures indicate that the increased cell surface half-life is due to pi-pi stacking interactions between Trp-167 of HLA-A2.1 and the aromatic P1 residues of I1F and I1Y. Comparison of the structures and modeling potential T cell receptor (TCR) interactions suggests that T cell interactions and immunogenicity are different between I1Y and I1F for two reasons. First, subtle changes in the steric and polar properties of the I1Y peptide affect TCR engagement. Second, water-mediated hydrogen bond interactions between the P1-Tyr and the P4-Glu peptide residues increase peptide side chain rigidity of residues critical for TCR engagement.

Circular Dichroism↗
PubMed2003

Enhanced immune presentation of a single-chain major histocompatibility complex class I molecule engineered to optimize linkage of a C-terminally extended peptide.

Major histocompatibility complex class I molecules can be expressed as single polypeptides wherein the antigenic peptide, beta2-microglobulin, and heavy chain are attached by flexible linkers. These molecules, single-chain trimers (SCTs), are remarkably stable at the cell surface compared with native (noncovalently attached) class I molecules. In this study, we used a structure-based approach to engineer an F pocket variant SCT of the murine class I molecule Kb that presents the SIINFEKL epitope of ovalbumin. Mutation of heavy chain residue Tyr84 (Y84A) in the SCT resulted in enhanced serological and cytolytic CD8 T cell recognition of the covalently linked peptide due to better accommodation of the linker extending from the C terminus of the peptide. These SCTs exhibit significant cell-surface stability, which we hypothesize is rendered by their ability to continuously and efficiently rebind the covalently attached peptide. In addition, we demonstrate that SCT technology can be applied to tetramer construction using recombinant SCTs expressed in Escherichia coli. SCT-based tetramers could have applications for the enumeration of T and natural killer cells that recognize peptide.class I complexes prone to dissociation.

Amino Acid Sequence↗
PubMed2006

Stable isotope tagging of epitopes: a highly selective strategy for the identification of major histocompatibility complex class I-associated peptides induced upon viral infection.

Identification of peptides presented in major histocompatibility complex (MHC) class I molecules after viral infection is of strategic importance for vaccine development. Until recently, mass spectrometric identification of virus-induced peptides was based on comparative analysis of peptide pools isolated from uninfected and virus-infected cells. Here we report on a powerful strategy aiming at the rapid, unambiguous identification of naturally processed MHC class I-associated peptides, which are induced by viral infection. The methodology, stable isotope tagging of epitopes (SITE), is based on metabolic labeling of endogenously synthesized proteins during infection. This is accomplished by culturing virus-infected cells with stable isotope-labeled amino acids that are expected to be anchor residues (i.e. residues of the peptide that have amino acid side chains that bind into pockets lining the peptide-binding groove of the MHC class I molecule) for the human leukocyte antigen allele of interest. Subsequently these cells are mixed with an equal number of non-infected cells, which are cultured in normal medium. Finally peptides are acid-eluted from immunoprecipitated MHC molecules and subjected to two-dimensional nanoscale LC-MS analysis. Virus-induced peptides are identified through computer-assisted detection of characteristic, binomially distributed ratios of labeled and unlabeled molecules. Using this approach we identified novel measles virus and respiratory syncytial virus epitopes as well as infection-induced self-peptides in several cell types, showing that SITE is a unique and versatile method for unequivocal identification of disease-related MHC class I epitopes.

Amino Acid Sequence↗
PubMed1992

Species specificity and augmentation of responses to class II major histocompatibility complex molecules in human CD4 transgenic mice.

Murine T cell responses to human class II major histocompatibility complex (MHC) molecules were shown to be a minimum of 20-70-fold lower than responses to allogeneic molecules. Transgenic mice expressing slightly below normal (75-95%) or very high (250-380%) cell surface levels of human CD4 were utilized to determine whether this was due to a species-specific interaction between murine CD4 and class II molecules. Human CD4 was shown to function in signal transduction events in murine T cells based on the ability of anti-human CD4 antibody to synergize with suboptimal doses of anti-murine CD3 antibody in stimulating T cell proliferation. In mice expressing lower levels of human CD4, T cell responses to human class II molecules were enhanced up to threefold, whereas allogeneic responses were unaltered. In mice expressing high levels of human CD4, responses to human class II molecules were enhanced at least 10-fold, whereas allogeneic responses were between one and three times the level of normal responses. The relatively greater enhancement of the response to human class II molecules in both lines argues for a preferential interaction between human CD4 and human class II molecules. In mice expressing lower levels of human CD4, responses to human class II molecules were blocked by antibodies to CD4 of either species, indicating participation by both molecules. In mice expressing high levels of human CD4, responses to both human and murine class II molecules were almost completely blocked with anti-human CD4 antibody, whereas anti-murine CD4 antibody had no effect. However, anti-murine CD4 continued to synergize with anti-CD3 in stimulating T cell proliferation in these mice. Thus, overexpression of human CD4 selectively impaired the ability of murine CD4 to assist in the process of antigen recognition. The ability of human CD4 to support a strong allogeneic response under these conditions indicates that this molecule can interact with murine class II molecules to a significant extent. Despite the fact that human CD4 appeared to be the only functional coreceptor in these mice, responses to human class II molecules were still much lower than those to murine class II alloantigens. This indicates that species-specific interactions between class II molecules and CD4 expressed on peripheral T cells are not sufficient to account for the low xenogeneic response and that intrinsic differences in T cell receptor structures or the need for species specificity in the interaction between CD4 and class II molecules during positive selection are also important.

Animals↗
PubMed1994

Availability of endogenous peptides limits expression of an M3a-Ld major histocompatibility complex class I chimera.

Taking advantage of our understanding of the peptide specificity of the major histocompatibility complex class I-b molecule M3a, we sought to determine why these molecules are poorly represented on the cell surface. To this end we constructed a chimeric molecule with the alpha 1 and alpha 2 domains of M3a and alpha 3 of Ld thereby allowing use of available monoclonal antibodies to quantify surface expression. Transfected, but not control, B10.CAS2 (H-2M3b) cells were lysed readily by M3a-restricted monoclonal cytotoxic T lymphocytes. Thus, the chimera bound, trafficked, and presented endogenous mitochondrial peptides. However, despite high levels of M3a-Ld mRNA, transfectants were negative by surface staining. This finding was consistent with inefficient trafficking to the cell surface. Incubation at 26 degrees C, thought to permit trafficking of unoccupied heavy (H) chains, resulted in detectable cell surface expression of chimeric molecules. Incubation with exogenous peptide at 26 degrees C (but not at 37 degrees C) greatly enhanced expression of M3a-Ld molecules in a dose-dependent manner, suggesting stabilization of unoccupied molecules. Stable association of beta 2-microglobulin with the chimeric H chain was observed in labeled cell lysates only in the presence of exogenous specific peptide, indicating that peptide is required for the formation of a ternary complex. These results indicate that surface expression of M3a-Ld is limited largely by the steady-state availability of endogenous peptides. Since most known M3a-binding peptides are N-formylated, native M3a may normally be expressed at high levels only during infection by intracellular bacteria.

Animals↗
PubMed1994

YB-1 DNA-binding protein represses interferon gamma activation of class II major histocompatibility complex genes.

Interferon gamma (IFN-gamma) is the most potent inducer of class II major histocompatibility complex (MHC) genes. This induction is uniquely mediated by three DNA elements in the promoter region of class II MHC genes. One of these DNA elements, Y, contains an inverted CCAAT box. Previously, we have screened a lambda gt11 library for Y-binding proteins and identified the YB-1 gene. Here we provide evidence that YB-1 can repress the IFN-gamma induction of class II MHC promoter as well as the Invariant chain (Ii) gene which also contains a Y element in its promoter. This was demonstrated by cotransfecting a YB-1 expression vector with promoter-reporter gene constructs. As an alternate approach, an efficient transient transfection system was developed which resulted in a > 70% transfection efficiency. Transfection of YB-1 by this procedure resulted in the near abrogation of IFN-gamma induced HLA-DR antigen and mRNA expression. These findings show the functional suppression of class II MHC gene induction by the YB-1 protein.

Animals↗
PubMed1994

Human major histocompatibility complex class II-restricted T cell responses in transgenic mice.

Transgenic mice expressing human major histocompatibility complex (MHC) class II molecules would provide a valuable model system for studying human immunology. However, attempts to obtain human class II-restricted T cell responses in such transgenic mice have had only limited success, possibly due to an inability of mouse CD4 to interact efficiently with human MHC class II molecules. To circumvent this problem, we constructed recombinant MHC class II genes in which the peptide-binding domain was derived from human DR sequences whereas the CD4-binding domain was derived from mouse I-E sequences. Purified chimeric human/mouse MHC class II molecules were capable of specifically binding DR-restricted peptides. Human B cell transformants that expressed these chimeric MHC class II molecules could present peptide antigens to human T cell clones. Expression of these chimeric class II molecules in transgenic mice led to the intrathymic deletion of T cells expressing superantigen-reactive V beta gene segments, indicating that the chimeric class II molecules could influence the selection of the mouse T cell repertoire. These transgenic mice were fully capable of mounting human DR-restricted immune responses after challenge with peptide or whole protein antigens. Thus, the chimeric class II molecules can serve as functional antigen presentation molecules in vivo. In addition, transgenic mice expressing chimeric class II molecules could be used to generate antigen-specific mouse T cell hybridomas that were capable of interacting with human antigen-presenting cells.

Animals↗
PubMed1997

Initiation codon scanthrough versus termination codon readthrough demonstrates strong potential for major histocompatibility complex class I-restricted cryptic epitope expression.

Accumulating evidence shows that the repertoire of major histocompatibility complex class I-restricted epitopes extends beyond conventional translation reading frames. Previously, we reported that scanthrough translation, where the initiating AUG of a primary open reading frame is bypassed, is most likely to account for the presentation of cryptic epitopes from alternative reading frames within the influenza A PR/8/34 nucleoprotein gene. Here, we confirm and extend these findings using an epitope cassette construct that features two well-defined CD8(+) T cell (TCD8+) epitopes in alternative reading frames, each preceded by a single start codon. Expression of one epitope depends on scanning of the ribosome over the first AUG with translation initiation occurring at the second AUG. We find that scanthrough translation has great potency in our system, with its impact being modulated, as predicted, by the base composition surrounding the first initiation codon, the number of start codons preceding the point of alternate reading frame initiation, and the efficiency with which the epitope itself is generated. Additionally, we investigated the efficiency of eukaryotic translation termination codons, to assess codon readthrough as a mechanism for cryptic epitope expression from 3' untranslated regions. In contrast with initiation codons, eukaryotic stop codons appear to be highly efficient at preventing expression of epitopes encoded in 3' untranslated regions, suggesting that 3' untranslated regions are not a common source of cryptic epitope substrate. We conclude that scanthrough is a powerful mechanism for the expression of epitopes encoded in upstream alternative open reading frames that may contribute significantly to TCD8+ responses and to tolerance induction.

Animals↗
PubMed2003

A new type of metal recognition by human T cells: contact residues for peptide-independent bridging of T cell receptor and major histocompatibility complex by nickel.

In spite of high frequencies of metal allergies, the structural basis for major histocompatibility complex (MHC)-restricted metal recognition is among the unanswered questions in the field of T cell activation. For the human T cell clone SE9, we have identified potential Ni contact sites in the T cell receptor (TCR) and the restricting human histocompatibility leukocyte antigen (HLA)-DR structure. The specificity of this HLA-DR-promiscuous VA22/VB17+ TCR is primarily harbored in its alpha chain. Ni reactivity is neither dependent on protein processing in antigen-presenting cells nor affected by the nature of HLA-DR-associated peptides. However, SE9 activation by Ni crucially depends on Tyr29 in CDR1alpha, an N-nucleotide-encoded Tyr94 in CDR3alpha, and a conserved His81 in the HLA-DR beta chain. These data indicate that labile, nonactivating complexes between the SE9 TCR and most HLA-DR/peptide conjugates might supply sterically optimized coordination sites for Ni ions, three of which were identified in this study. In such complexes Ni may effectively bridge the TCR alpha chain to His81 of most DR molecules. Thus, in analogy to superantigens, Ni may directly link TCR and MHC in a peptide-independent manner. However, unlike superantigens, Ni requires idiotypic, i.e., CDR3alpha-determined TCR amino acids. This new type of TCR-MHC linkage might explain the high frequency of Ni-reactive T cells in the human population.

Amino Acid Sequence↗
PubMed2003

Are major histocompatibility complex molecules involved in the survival of naive CD4+ T cells?

The exact role of major histocompatibility complex (MHC) molecules in the peripheral survival of naive T cells is controversial, as some studies have suggested that they are critically required whereas others have suggested that they are not. Here we controlled for some of the features that differed among the earlier studies, and analyzed both the survival and expansion of naive CD4+ T cells transferred into MHC syngeneic, allogeneic, or MHC negative environments. We found that naive T cells transferred into MHC negative or allogeneic environments often fail to survive because of rejection and/or competition by natural killer (NK) cells, rather than failure to recognize a particular MHC allele. In the absence of NK cells, naive CD4+ T cells survived equally well regardless of the MHC type of the host. There was, however, an MHC requirement for extensive space-induced "homeostatic" expansion. Although the first few divisions occurred in the absence of MHC molecules, the cells did not continue to divide or transit to a CD44hi phenotype. Surprisingly, this MHC requirement could be satisfied by alleles other than the restricting haplotype. Therefore, space-induced expansion and survival are two different phenomena displaying different MHC requirements. Memory CD4+ T cells, whose survival and expansion showed no requirements for MHC molecules at all, dampened the space-induced expansion of naive cells, showing that the two populations are not independent in their requirements for peripheral niches.

Alleles↗
PubMed1996

Malignant potential, major histocompatibility complex antigen expression, and genomic imprinting of rat trophoblast cell lines.

Invasive growth, variation in major histocompatibility complex antigen expression, and genomic imprinting are important properties of both trophoblast cells and malignant tumors. This study, undertaken to address these three issues, used cultured trophoblast cell lines derived from Day 11/12 rat placentas of all mating combinations of the DA and WF inbred strains. In addition, genomic imprinting was also examined in intact rat placentas from Days 11-19. There was no correlation in trophoblast cells between class I antigen expression, DNA content, and cell ploidy on the one hand and oncogenic potential on the other hand. The constitutive suppression of class II antigens in the trophoblast cells could not be abrogated by treatment with interferon-gamma, whereas such treatment always maximally induced class I antigen expression regardless of the initial resting levels. The trophoblast cells at Day 11/12 expressed both maternal and paternal class I antigens, and studies in whole placental tissues showed that the imprinting of the maternal class I antigens was manifested by a decreased level of expression rather than an absence of expression. Thus, genomic imprinting in the rat placenta is a quantitative, rather than an all-or-none, phenomenon.

Animals↗
PubMed1992

Ultrastructural location of major histocompatibility complex (MHC) class II positive perivascular cells in histologically normal human brain.

The expression of major histocompatibility complex (MHC) class I and II antigens was studied in surgical and postmortem brain biopsy tissue using light and electron microscopic immunocytochemistry. In addition, monoclonal antibodies directed against human macrophages (EBM11) and alpha-smooth muscle actin were applied. It is shown that blood vessel-associated MHC class II immunoreactivity in histologically normal human brain can be localized to a distinct class of cells, termed perivascular cells, which share macrophage but not smooth muscle cell antigen. This immunophenotype, the location in the perivascular space as well as the morphology, frequency and tissue distribution distinguish perivascular cells from pericytes and intraparenchymal microglia. It is suggested that MHC class II positive perivascular cells are a normal constituent of the human cerebral microvasculature. The potential role of these cells in immunological reactions occurring at the blood-brain interface is discussed.

Antibodies, Monoclonal↗
PubMed1987

Expression of class II major histocompatibility complex antigens by bronchial epithelium in rat lung allografts.

Variations in expression of class II major histocompatibility complex antigens on bronchial epithelial cells and vascular endothelium were investigated in normal rat lungs and allografted lungs during acute rejection and after cyclosporine (CsA) treatment. BN (RT1n) left lungs were transplanted into LEW (RT1l) recipients. Lungs were excised during acute rejection in untreated rats on postoperative days 1 through 5, and after CsA treatment (25 mg/kg on days 2 and 3) on days 5 and 100. Cryostat sections were examined for class II antigen expression with an immunoperoxidase technique, using various monoclonal antibodies. In the normal lung, class II antigens were not expressed by epithelial or endothelial cells. In the allografts, induction of class II antigens closely correlated with the rejection process: on day 2, the ciliated bronchial epithelium was locally positive; it became uniformly positive with increasing cellular peribronchial infiltration on days 3 and 4. CsA treatment prevented class II antigen expression to a certain extent, leaving the bronchial epithelium weakly positive at 100 days. Endothelial cells were invariably negative for class II antigens in all allografted lungs. The class II antigens expressed on the bronchial epithelial cells were of graft origin, except for recipient-type class II molecules found on the ciliated surface in CsA-treated animals. We conclude that expression of class II antigens by bronchial epithelium is the result of a bronchus-directed rejection process, and hypothesize that such a rejection process may have caused bronchiolitis obliterans in several of the patients with combined heart-lung transplants. Important is the observation that class II molecules can be present on the membranes of cells that do not themselves produce these antigens.

Animals↗
PubMed1991

Relative ability of distinct isotypes of human major histocompatibility complex class II molecules in binding staphylococcal enterotoxin A.

Relative ability of distinct isotypes of human major histocompatibility complex class II molecules to bind staphylococcal enterotoxin A (SEA) was investigated. SEA-binding was observed in L cells transfected with DR2 and DQw1 genes. By contrast, it was not detected in L cells transfected with DPw4 and DP (Cp63) genes. All the transfectants supported SEA-induced IL-2 production by human T cells. Levels of the accessory activity were low in the DPw4 and DP (Cp63) transfectants compared with the DR2 and DQw1 transfectants. In view of the observation that all the transfectants express well the transfected gene products on their surface, the results indicate that DR and DQ molecules bind SEA with high affinity, while DP molecules bind it with fairly low affinity.

Animals↗
PubMed2006

Chicken major histocompatibility complex class II molecules of the B haplotype present self and foreign peptides.

The chicken major histocompatibility complex (MHC), or B-complex, mediates genetic resistance and susceptibility to infectious disease. For example, the B19 haplotype is associated with susceptibility to Marek's disease. Here, we describe the sequencing and analysis of peptides presented by B19 MHC class II molecules. A B19/B19 B-cell line was used for the immunoaffinity purification of MHC class II molecules, which was followed by acid elution of the bound peptides. The eluted peptides were then analysed using tandem mass spectrometry. Thirty peptide sequences were obtained, ranging from 11 to 25 amino acids in length. Source protein cellular localization included the plasma membrane, cytosol and endosomal pathway. In addition, five peptides from the envelope glycoprotein of chicken syncytial virus (CSV) were identified. Chicken syncytial virus had been used as a helper virus along with reticuloendotheliosis virus strain T for transformation of B19/B19B cells. Alignment and analysis of the peptide sequence pool provided a putative peptide-binding motif for the B19 MHC class II.

Amino Acid Motifs↗
PubMed2006

Regulation of class I major histocompatibility complex receptor expression in natural killer cells: one promoter is not enough!

The class I major histocompatibility complex (MHC) receptors expressed by natural killer (NK) cells play an important role in regulating their function. The number and type of inhibitory receptors expressed by NK cells must be tightly controlled in order to avoid the generation of dominantly inhibited NK cells. The selective stochastic expression of the class I MHC receptors generates a variegated NK cell population capable of discriminating subtle changes in MHC expression on potential target cells. The molecular mechanisms controlling the cell-specific and probabilistic expression of these receptors are without doubt very complex. The traditional approach of considering a core promoter modulated by upstream enhancer elements is likely too simplistic a paradigm to adequately explain the regulation of these genes, as well as other gene clusters that are not expressed in an 'all or none' fashion. Our studies on the regulation of the mouse Ly49 and human killer immunoglobulin-like receptor (KIR) clusters of class I MHC receptor genes have revealed the presence of multiple transcripts in both sense and antisense orientations. In both systems, an antisense promoter overlaps a promoter that produces sense transcripts, creating a bidirectional element. In the Ly49 genes, the competing promoters behave as probabilistic switches, and it is likely that the human bidirectional promoters will have a similar property. The antisense transcripts generated in the Ly49 genes are far removed from the promoter responsible for Ly49 expression in mature NK cells, whereas the antisense KIR transcripts detected are within the adult promoter region. This finding suggests that the mechanism of promoter regulation in the KIR genes may be quite different from that of the Ly49 genes. This review summarizes the current state of knowledge regarding class I MHC receptor gene regulation. The models proposed for the control of the probabilistic expression of the Ly49 and KIR genes are discussed in the context of current knowledge regarding the complex control of other well-studied gene clusters such as the beta-globin and cytokine clusters.

Animals↗
PubMed1990

Expression of class II major histocompatibility complex antigen on mouse sperm and its roles in fertilization.

The expression of class II major histocompatibility complex (MHC) antigen on the membrane of mouse sperm head was clearly demonstrated by indirect immunofluorescence (IIF) test, enzyme immunoassay (EIA), and mixed lymphocyte sperm reaction (MLSR) as well as the examination of its expression at transcriptional levels by northern dot blotting. Furthermore, the roles of class II MHC antigen of sperm in fertilization were investigated with an in vitro fertilization (IVF) system. The successful ratio of IVF was significantly decreased by treatment of sperm with anticross-reactive region of class II MHC antigen monoclonal antibody (MAb) but not with antiprivate region MAb. These results were not due to disturbances of sperm mobility by these MAbs. It was strongly suggested that the monomorphic or its related region of class II MHC antigen on sperm played an important role in the recognition between sperm and egg in fertilization.

Animals↗