Rules for Peptide Presentation by MHC Class II Molecules

Molecules of the major histocompatibility complex (MHC) were first discovered for their importance in the Rejection of tissue grafts between unrelated individuals. If these individuals differ at one or more loci within the MHC, the grafts are Rapidly Rejected. The Rejection process involves Recognition of foreign MHC molecules by T lymphocytes and the Rapid destruction by these cells of the grafted tissue cells. The MHC is a highly polymorphic complex of genes, making tissue grafting between unrelated individuals virtually impossible. Thus, it stimulated a great deal of work when organ transplantation became technically feasible.     

High Peptide Affinity for MHC Class I does not Correlate with Immunodominance

Cytotoxic T (Tc)-cell responses against influenza virus infection in BALB/c (H-2d) mice are dominated by Tc clones reactive to the viral nucleoprotein (NP). Here, we report investigations using recombinant vaccinia viruses (VV) encoding major histocompatibility complex (MHC) class I H-2Kd molecules differing by a single amino acid from glutamine (wild-type, Kdw) to histidine (mutant, Kdm) at position 114 located in the floor of the peptide-binding groove. Influenza-infected target cells expressing Kdw were strongly lysed by Kd-restricted Tc cells against A/WSN influenza virus or the immunodominant peptide of viral NP (NPP147-155), whereas infected Kdm-expressing targets gave little or no lysis, respectively, thus showing the immunodominance of NPP147-155. Kdm-expressing target cells saturated with synthetic NPP147-155 (10-5 M) were lysed similarly to Kdw-expressing targets by NPP147-155-specific Tc cells. Thus the defect in influenza-infected Kdm-expressing targets was quantitative; insufficient Kdm-peptide complexes were expressed. Tc-cell responses against four other viruses or alloantigens showed no effect of Kdm. When peptide transport-defective cells were infected with VV-Kdw or VV-Kdm and co-infected with a recombinant VV encoding an endoplasmic reticulum-targeted viral peptide, two influenza haemaglutinin peptides caused higher expression of Kdw than NPP147-155 indicating their higher affinity for Kdw. These results are inconsistent with the hypothesis that immunodominance in the anti-influenza response reflects high affinity of the immunodominant peptide, but are consistent with skewing of the Tc-cell receptor repertoire.     

Peptide sequences binding to MHC class I proteins

Motifs for peptides which bind specifically to the human class I major histocompatibility complex molecules HLA-A2 and B7 were determined by sequence analysis of class I-bound peptides selected from a random synthetic library of nonamers. Thirteen individual peptides were sequenced for HLA-A2, twelve individual and nine pooled peptides were sequenced for HLA-B7. Analysis of sequence alignment implicated four peptide positions in potential contact with the class I HLA-A2 molecule and three positions for the HLA-B7 molecule. The results demonstrate that a synthetic peptide library can be used to identify allele-specific motifs for class I molecules, providing information comparable to the results obtained from sequencing endogenous peptides. This method utilizes denatured class I heavy chains, and similar results were obtained using a class I protein purified from mammalian cells or by expression in Escherichia coli. This method has the potential to detect peptides which may not be generated physiologically, but due to their binding properties, may be valuable to predict or engineer immunomodulatory T cell epitopes.     

MHC Class II-Dependent Peptide Antigen Versus Superantigen Presentation to T Cells

T lymphocytes expressing the CD4 coreceptor can be activated by two classes of major histocompatibility complex (MHC) class II-bound ligands. The elaboration of a conventional T-cell mediated immune response involves recognition of an antigenic peptide bound to the MHC class II molecules by a T-cell receptor (TCR) specific to that particular antigen. Conversely, superantigens (SAgs) also bind to MHC class II molecules and activate T cells, leading to a completely different functional outcome; indeed, SAg-responsive T cells die through apoptosis following stimulation. Superantigens are proteins that are secreted by various bacteria. They interact with the TCR using molecular determinants that are distinct from the residues involved in the recognition of nominal antigenic peptides. Despite the similarities between the recognition of the two classes of ligands by the TCR, considerable structural difference is observed. Here, we discuss the current knowledge on the presentation of SAgs to T cells and compare the different aspects of the SAg response with the recognition of antigenic peptide/MHC complexes.     

Peptide Binding to Class I Molecules of the Major Histocompatibility Complex on the Surface of Living Target Cells

Molecules encoded by the class I major histocompatibility genes bind short (nonameric) peptides produced by intracellular proteolysis of antigens. These complexes formed intracellularly are then expressed on membranes of target cells and recognized by the antigen receptor of cytolytic T cells. No binding of externally added peptides could so far be monitored directly on the antigen presenting cells, although cytotoxicity experiments and indirect binding assays provided evidence for its existence. Here we report experiments where specific binding to class I molecules, of externally added peptides, has been monitored on living cells. N-terminal biotin-labelled Kd-restricted peptides (residues 147-155, residues 147-158, and an analogue lacking the arginine at position 156, derived from the sequence of the influenza A virus nucleoprotein) were incubated with murine H-2Kd mastocytoma cells (line P815) at 4 degrees C. The binding on surface of live, intact cells was then demonstrated fluorometrically via the interaction of a streptavidin-phycoerythrin conjugate with the biotin-labelled peptides. Thus, this binding does not involve processing, and its specificity in terms of peptide structure was established by competition with the respective unmodified peptides. The specificity of binding to class I molecules was demonstrated by blocking experiments using monoclonal antibodies specific for H-2Kd. Finally, a correlation was observed between the results of peptide binding measurements and those of cytotoxicity assays.     

TCR-like Biomolecules Target Peptide/MHC Class I Complexes on the Surface of Infected and Cancerous Cells

The human leukocyte antigen (HLA; also called major histocompatibility, or MHC) class I system presents peptides that distinguish healthy from diseased cells. Therefore, the discovery of peptide/MHC class I markers can provide highly specific targets for immunotherapy. Over the course of almost two decades, various strategies have been used, with mixed success, to produce antibodies that have recognition specificity for unique peptide/MHC class I complexes that mark infected and cancerous cells. Using these antibody reagents, novel peptide/MHC class I targets have been directly validated on diseased cells and new insight has been gained into the mechanisms of antigen presentation. More recently, these antibodies have shown promise for clinical applications such as therapeutic targeting of cancerous and infected cells and diagnosis and imaging of diseased cells. In this review, the authors comprehensively describe the methods used to identify disease-specific peptide/MHC class I epitopes and generate antibodies to these markers. Finally, they offer several examples that illustrate the promise of using these antibodies as anti-cancer agents.     

Delivery of Antigens to the MHC Class I Pathway Using Bacterial Toxins

Cytotoxic T lymphocytes (CTL) recognize antigens derived from endogenously expressed proteins presented on the cell surface in the context of major histocompatibility complex (MHC) class I molecules. Because CTL are effective in antiviral and antitumor responses, the delivery of antigens to the class I pathway has been the focus of numerous efforts. Generating CTL by immunization with exogenous proteins is often ineffective because these antigens typically enter the MHC class II pathway. This review focuses on the usefulness of bacterial toxins for delivering antigens to the MHC class I pathway. Several toxins naturally translocate into the cytosol, where they mediate their cytopathic effects, and the mechanisms by which this occurs has been elucidated. Molecular characterization of these toxins identified the functional domains and enabled the generation of modified proteins that were no longer toxic but retained the ability to translocate into the cytosol. Thus, these modified toxins could be examined for their ability to carry peptides or whole proteins into the cytosolic processing pathway. Of the toxins studied—diphtheria, pertussis, Pseudomonas, and anthrax—the anthrax toxin appears the most promising in its ability to deliver large protein antigens and its efficiency of translocation.     

A 21-Mer Synthetic Peptide of Toxic Shock Syndrome Toxin 1, TSST-1[58-78], Activates T Cells by Binding to MHC Class II and by an MHC Unrestricted Xenostimulatory Pathway

Toxic shock syndrome toxin-1 (TSST-1) is a “superantigen” which binds to MHC class II molecules and induces a polyclonal stimulation of T cells. In this communication by using a FACS technique and a 21-mer synthetic peptide from the primary sequence of TSST-1 (KGEKVDLNTKRTKKSQHTSEG), designated TSST-1 (58-78), we demonstrated binding of the peptide only to cells bearing MHC class II. The proliferative effect of TSST-1 (58-78) on human T cells was shown to be inhibited much more by anti-HLA-DR than by anti-HLA class I antibody. Furthermore, human monocytes were able to present TSST-1 (58-78) to a mouse VSV specific T cell clone by a xenostimulatory mechanism. These data indicate this peptide to contain an active site of the TSST-1 holotoxin.     

MHCⅠ类基因转移诱导移植免疫耐受的研究

本研究用体内外实验观察MHC Ⅰ类基因转移对移植物免疫原性的影响。并用小鼠心肌移植模型, 探讨通过MHCⅠ类基因转移延长移植物存活的可行性。结果表明, 正常对照组小鼠的心肌移植物平均存活时间(MST) 为9-4d±1-84d。诱导耐受组小鼠的心肌移植物MST为45-7d±6-0d, 比对照组明显延长( P<0-05) 。提示将供体MHC Ⅰ类基因转移到受体造血细胞, 可以降低移植物的免疫原性, 延长移植物的存活时间     

Impact of MHC Class I Gene on Resistance to Murine AIDS

Development of murine AIDS in mice following infection with LP-BM5 murine leukaemia virus (MuLV) is highly strain dependent, with strain differences determind by genes within and outside H-2. Among H-2 genes, the D^d gene is the most closely associated with resistance to LP-BM5 MuLV infection. However, the D^d-mediated resistance is highly influenced by outside H-2 genes, i. e. A lineage strains are more resistant than mice strains of B6/B10 lineage. In this study, the mice having BALB background were analysed and, similarly to A lineage mice, only D^d gene products were found to be required to provide resistance to LP-BM5 MuLV infection. Furthermore, BALB/c Kh mice bearing both D^d and L^d genes clearly showed obviously higher resistance than BALB/c-H-2^dm2 mice solely having the D^d gene. In addition, in the long-term observation of the effect of the D^d gene on B6/B10 background mice, D8 mice having the D^d gene as a transgene and expressing a high level D^d gene product showed higher resistance than naturally recombinant B10. A(18R) mice. These results suggest that the MAIDS resistance associated with the D end loci is dependent on the level of expression of an MHC class I gene.     

Peptide binding to MHC class I molecules: Implications for antigenic peptide prediction

The human mayor histocompatibility complex class I molecule HLA-A2 preferentially binds peptides that contain Leu at P2 and Val or Leu at the C terminus. The other amino acids in the peptide also contribute to binding positively or negatively. It is possible to estimate the binding stability of HLA-A2 complexes containing particular peptides by applying coefficients, deduced from a large amount of binding data, that quantify the relative contribution of each amino acid at each position. In this review, we describe the molecular basis for these coefficients and demonstrate that estimates of binding stability based on the coefficients are generally concordant with experimental measurements of binding affinities. Peptides that contained cysteine were predicted less well, possibly because of complications resulting from peptide dimerization and oxidation. Apparently, peptide binding affinity is largely controlled by the rate of dissociation of the β₂-Microglobulin complex, whereas the rate of formation of the complex has less impact on peptide affinity. Although peptides that bind tightly to HLA-A2, including many antigenic peptides bind much more weakly. Therefore, a full understanding of why certain peptides are immunodominant will require further research.     

Targeting antigen to MHC Class I and Class II antigen presentation pathways for malaria DNA vaccines

An effective malaria vaccine which protects against all stages of Plasmodium infection may need to elicit robust CD8(+) and CD4(+) T cell and antibody responses. To achieve this, we have investigated strategies designed to improve the immunogenicity of DNA vaccines encoding the Plasmodium yoelii pre-erythrocytic stage antigens PyCSP and PyHEP17, by targeting the encoded proteins to the MHC Classes I and II processing and presentation pathways. For enhancement of CD8(+) T cell responses, we targeted the antigens for degradation by the ubiquitin (Ub)/proteosome pathway following the N-terminal rule. We constructed plasmids containing PyCSP or PyHEP17 genes fused to the Ub gene: plasmids where the N-terminal antigen residues were mutated from the stabilizing amino acid methionine to destabilizing arginine, plasmids where the C-terminal residues of Ub were mutated from glycine to alanine, and plasmids in which the potential hydrophobic leader sequences of the antigens were deleted. For enhancement of CD4(+) T cell and antibody responses, we targeted the antigens for degradation by the endosomal/lysosomal pathway by linking the antigen to the lysosome-associated membrane protein (LAMP). We found that immunization with DNA vaccine encoding PyHEP17 fused to Ub and bearing arginine induced higher IFN-gamma, cytotoxic and proliferative T cell responses than unmodified vaccines. However, no effect was seen for PyCSP using the same targeting strategies. Regarding Class II antigen targeting, fusion to LAMP did not enhance antibody responses to either PyHEP17 or PyCSP, and resulted in a marginal increase in lymphoproliferative CD4(+) T cell responses. Our data highlight the antigen dependence of immune enhancement strategies that target antigen to the MHC Class I and II pathways for vaccine development.