Physical association between MHC class I molecules and immunogenic peptides

In vitro peptide binding to the heavy chain of the class I molecule of the major histocompatibility complex molecule HLA-A2

The heavy chain of class I molecules of the major histocompatibility complex forms the binding site for antigenic peptides. We describe the binding of a synthetic peptide to the purified heavy chain of the human major histocompatibility complex molecule HLA-A2. The peptide binding capacity is found to be markedly increased if the protein is first partly denatured by reduction of its disulfide bonds in detergent and subsequently renatured by reoxidation. In the presence of certain detergents, the heavy chain binds peptides even when the protein is partly unfolded.

A single-chain murine class I major transplantation antigen

Single-chain mouse Kd molecules (SC-Kd) were engineered by connecting residue 276 of Kd heavy chain to the first residue of beta 2-microglobulin through spacers of various lengths, and expressed intracellularly in monkey COS-1 cells. Labeled SC-Kd molecules were found to react with several monoclonal antibodies which recognize native Kd molecules. SC-Kd-15 (with a spacer of 15 residues) was studied in more details. It could be purified and shown to regain a native-like structure after treatment with denaturing agents. Purified SC-Kd-15 could bind certain peptides in a manner qualitatively similar to the Kd.

Pathways of antigen processing

Separate pathways exist for the processing of antigens to be presented by MHC class I and class II molecules. We are beginning to determine the subcellular location of certain events in both pathways.

A single amino acid substitution in the Ak molecule fortuitously provokes an alloresponse

We discovered by chance that the R28 T cell hybridoma has dual specificity. It responds to a peptide derived from ribonuclease presented by cells displaying Ak molecules and it reacts, in the absence of added antigen, to cells expressing Ak complexes with a single amino acid substitution at position 69 of the alpha chain. Modelling and functional studies suggest that residue 69 is a peptide contact residue, prompting the hypothesis that R28's alloreactivity is a cross-reactive response to an unknown peptide bound in the 'groove' of the mutant Ak complex. In this report, we employ a competition assay to confirm that this alloresponse involves a groove-binding peptide, demonstrate that this peptide derives from or depends on fetal calf serum and exploit a panel of antigen-presenting cell lines--each displaying an Ak complex with a different position 69 substitution--to establish that the alloresponse is not just a heteroclitic response to ribonuclease, itself. We speculate that much of the alloreactivity against murine class II molecules that is revealed in vitro may prove to be directed at bovine serum-derived peptides, suggesting that in this context, alloreactivity is a misnomer.

Expression of MHC-class-I antigens in human carcinomas and sarcomas analyzed by isoelectric focusing

We analyzed the expression of class-I antigens in ex vivo human tumor cells by isoelectric focusing (IEF) the anti-class-I mAb W6/32 immunoprecipitates prepared from cell lysates. Out of 42 experiments, 27 were technically successful. The patient's blood lymphocytes were used as controls. In vitro exposure of the tumor cells to IFN gamma and TNF alpha elevated class-I antigen expression. In 11 cases, defects in MHC-class-I-antigen expression were observed. In 2 cases the antigens were detected only in the cytokine-treated tumor samples, probably due to a defect in the association between beta 2m and class-I heavy chains. Selective changes in the expression of alleles were seen in 10 cases and might involve HLA A, B and C antigens. Alterations in class-I expression as compared with the lymphocytes were observed in 9 of 13 cases in which the tumor cells were collected from metastases, and only in 2 of 14 primary tumors.

E3/19K from adenovirus 2 is an immunosubversive protein that binds to a structural motif regulating the intracellular transport of major histocompatibility complex class I proteins

We have previously expressed in transgenic mice a chimeric H-2Kd/Kk protein called C31, which contains the extracellular alpha 1 domain of Kd, whereas the rest of the molecule is of Kk origin. This molecule functions as a restriction element for alloreactive and influenza A-specific cytotoxic T lymphocytes (CTL) but is only weakly expressed at the cell surface of splenocytes. Here, we show that the low cell surface expression is the result of slow intracellular transport and processing of the C31 protein. A set of hybrid molecules between Kd and Kk were used to localize the regions in major histocompatibility complex (MHC) molecules that are important for their intracellular transport and to further localize the structures responsible for binding to the adenovirus 2 E3/19K protein. This protein appears to be an important mediator of adenovirus persistence. It acts by binding to the immaturely glycosylated forms of MHC class I proteins in the endoplasmic reticulum (ER), preventing their passage to the cell surface and thereby reducing the recognition of infected cells by virus-specific T cells. We find the surprising result that intracellular transport and E3/19K binding are controlled primarily by the first half of the second domain of Kd, thus localizing these phenomena to the five polymorphic residues in this region of the Kd protein. This result implies that the E3/19K protein may act by inhibiting peptide binding or by disrupting the oligomerization of MHC class I molecules required for transport out of the ER. Alternatively, the E3/19K protein may inhibit the function of a positively acting transport molecule necessary for cell surface expression of MHC class I molecules.

A nucleoprotein peptide of influenza A virus stimulates assembly of HLA-B27 class I heavy chains and beta 2-microglobulin translated in vitro

Most cytotoxic T lymphocytes (CTL) recognize epitopes of foreign viral proteins in association with class I major histocompatibility complex (MHC) molecules. Viral proteins synthesized in the cytoplasm require intracellular fragmentation and exposure to the class I antigens for the development of CTL responses. Although indirect evidence for binding of peptides to class I antigens has accumulated, direct binding has only been shown recently. The formation of complexes between peptide and class I antigen may occur in the endoplasmic reticulum (ER) and peptides have been shown to induce assembly of the class I complex. We have translated the messenger RNAs encoding HLA-B27 (subtype 2705) and beta 2-microglobulin in a rabbit reticulocyte lysate supplemented with human microsomal membranes (to mimic ER membranes), in the absence and presence of a peptide derived from the nucleoprotein (residues 384-394) of influenza A virus. This peptide induces CTL activity against target cells expressing the HLA-B27 antigen. Here we report direct evidence that the nucleoprotein peptide promotes assembly of the HLA-B27 heavy chain and beta 2-microglobulin, and that this can occur in the ER immediately after synthesis of the two proteins.

Isolation of an endogenously processed immunodominant viral peptide from the class I H-2Kb molecule

Using an approach for isolating and characterizing peptide fractions that are intracellularly associated with major histocompatibility complex class I molecules, the major peptide recognized by cytotoxic T cells specific for the vesicular stomatitis virus has been isolated from the H-2Kb molecule of infected cells. This endogenously processed octapeptide is allele-specific as it does not bind to H-2Db molecules, and contains the core sequence of the epitope of the nucleocapsid protein of the vesicular stomatitis virus identified by testing with exogenous synthetic peptides.

In vitro human cytotoxic T cell responses against influenza A virus can be induced and selected by synthetic peptides

Studies on human anti-influenza cytolytic activities have demonstrated that cytotoxic T lymphocytes (CTL) from HLA-B37 individuals react preferentially with the peptide corresponding to residues 335-349 of the nucleoprotein, whereas CTL from HLA-A2 donors recognize peptide 57-68 from the viral matrix as a dominant epitope. We studied the secondary CTL response, obtained from peripheral blood mononuclear cells, of an HLA-A2+,B37+ individual stimulated either by infectious virus or by synthetic peptides. Only an HLA-B37-restricted response was detected after stimulation by the whole virus, showing an immunodominance of this activity over that restricted by HLA-A2. Moreover, human cytotoxic cell lines were successfully obtained after stimulation of peripheral blood mononuclear cells with synthetic peptides. Under these conditions, it was possible to selectively reveal the existence of an HLA-A2-restricted activity directed against the matrix peptide. These results demonstrate that, at least in vitro, it is possible to stimulate a latent repertoire by using synthetic peptides. Nevertheless, we could not induce a response against the matrix or the nucleoprotein peptides in HLA-A2- or B37- individuals, suggesting that a finer selection of synthetic peptides would be necessary for their possible utilization to induce CTL during vaccination.

Apparent lack of MHC restriction in binding of class I HLA molecules to solid-phase peptides

The specificity of binding of solubilized, purified HLA-A,B molecules to solid-phase peptides has been examined using the assay described by Bouillet et al. [1989. Nature (Lond.). 339:473.] 64 peptides derived from the sequences of viral antigens, HLA-A,B,C heavy chains, and clathrin light chains were tested for binding to HLA-A2.1, Aw68.1, Aw69, B44, and B5, molecules that differ by 5-17 residues of the peptide binding groove. 15 of the peptides, including those known to be T cell epitopes, gave significant binding. The pattern of peptide binding for each of the five HLA-A,B molecules was not significantly different. Binding was demonstrated to be a property of native beta 2m-associated HLA-A,B molecules that preserved conformational antigenic determinants after binding to peptide. In comparison to our previous results from solution-based assays the proportion of HLA-A,B molecules that can bind solid-phase peptides is very high. This accessibility of solid-phase peptides to the binding site of MHC molecules may be directly related to the observed absence of MHC specificity in the binding.

Evidence of widespread binding of HLA class I molecules to peptides

We have tested the binding of HLA class I proteins to peptides using a solid-phase binding assay. We tested 102 peptides, mostly derived from the HIV gag and HIV pol sequences. Most peptides did not bind to any class I protein tested. The pattern of binding among the three class I proteins tested, HLA-A2, -B27, and -B8, was approximately 85% concordant. Further, all five of the known HIV-1 gag T cell epitopes detected by human CTL bound at least one class I protein. Binding of class I to the peptides could be detected either by directly iodinated class I proteins, or indirectly using monoclonal antibodies specific for class I. The binding to the plates could be blocked with MA2.1, which binds in the alpha 1 region of A2, but not by W6/32, which binds elsewhere. The data presented here show that binding of class I to peptides is specific, but that many peptides bind to more than a single class I protein.

Analysis of physical interactions between peptides and HLA molecules and application to the detection of human immunodeficiency virus 1 antigenic peptides

The physical association of 40 antigenic peptides and purified HLA class I and class II molecules was monitored using a direct peptide binding assay (PBA) in solid phase and an inhibition peptide binding assay (IPBA) in which the competing peptide was present in a soluble phase. We also examined the ability of different peptides to inhibit the lytic activity of human antiviral cytolytic T cells towards cells incubated with the corresponding target peptide. Our results showed that: (a) Binding of a given human T cell-recognized peptide to several HLA class I and class II molecules occurred frequently. Nevertheless, preferential binding of peptides to their respective restriction molecules was also observed. (b) Binding of HLA molecules to peptides recognized by murine T cells occurred less frequently. (c) 11 of 24 (46%) randomly selected HIV-1 peptides contained agretopic residues allowing their binding to HLA molecules. (d) The kinetics of HLA/peptide association depended on the peptide tested and were faster than or similar to those reported for Ia molecules. Dissociation of these complexes was very low. (e) Peptide/HLA molecule binding was dependent on length, number of positive charges, and presence of hydrophobic residue in the peptide. (f) A correlation was demonstrated between a peptide inhibitory effect in the IPBA and its blocking effect in the cytolytic test. Our data indicated that the restriction phenomenon observed in T cell responses was not strictly related to either an elective HLA/peptide association, or a high binding capacity of a peptide to HLA molecules. These data also showed that the PBA and IPBA are appropriate for the detection of agretopic residues within HIV-1 proteins.

HLA-B37 and HLA-A2.1 molecules bind largely nonoverlapping sets of peptides

T-cell recognition of peptides that are bound and presented by class I major histocompatibility complex molecules is highly specific. At present it is unclear what role class I peptide binding plays relative to T-cell receptor specificity in determination of immune recognition. A previous study from our group demonstrated that the HLA-A2.1 molecule could bind to 25% of the members of a panel of unrelated synthetic peptides as assessed by a functional peptide competition assay. To determine the peptide-binding specificity of another HLA class I molecule, we have examined the capacity of this panel of peptides to compete for the presentation of influenza virus nucleoprotein peptide NP-(335-350) by HLA-B37 to NP-peptide-specific HLA-B37-restricted cytotoxic T-lymphocyte lines. Forty-two percent of peptides tested were capable of inhibiting NP-(335-350) presentation by HLA-B37. Remarkably, none of these HLA-B37-binding peptides belong to the subset that was previously shown to bind to the HLA-A2.1 molecule. Only the NP-(335-350) peptide was capable of binding to both HLA-A2.1 and HLA-B37. These findings demonstrate that the peptide-binding specificities of HLA-B37 and HLA-A2.1 are largely nonoverlapping and suggest that, from the universe of peptides, individual HLA class I molecules can bind to clearly distinct subsets of these peptides.

Guilt by association: HLA-B27 and ankylosing spondylitis

The remarkable association between HLA-B27 and ankylosing spondylitis (AS) remains an enigma. While previous reviews have discussed the controversies surrounding the involvement of bacteria in the etiology of this disease and the sequence variability between subtypes of HLA-B27, concepts of disease mechanism remain ill-defined. In this article Richard Benjamin and Peter Parham synthesize new data on the structure and function of HLA class I molecules into possible mechanisms that might underly the pathogenesis of AS.

Peptide ligand-induced conformation and surface expression of the Ld class I MHC molecule

Newly synthesized major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum are thought to bind peptides of foreign and endogenous antigens. Several lines of evidence indicate that beta-2 microglobulin (beta 2m) and/or peptide ligand participate in the intracellular transport and surface expression of class I molecules, but the nature of their involvement is still unclear. Here we present evidence that culturing non-mutant cells (fibroblast, thymoma or mastocytoma) with a peptide ligand specific for the Ld class I molecule of the mouse leads to a dramatic (fourfold) and specific induction of Ld surface expression. Surprisingly, this peptide ligand-induced expression of Ld does not result in an increased intracellular association of Ld with beta 2m. These findings demonstrate that the previously reported decrease in surface expression of Ld results from its failure to be saturated with endogenous self-peptide ligands. This unique feature of Ld could also contribute to the fact that several virus-specific cytotoxic T cell responses have been found to be Ld-restricted.

Inhibition of antigen presentation in vitro and in vivo by MHC antagonist peptides

A series of analogue peptides have been generated, using as a template the core region of the OVA 323-339 peptide identified as critical in determining binding to I-Ad. Several of these "core extended" peptides had increased affinities for the I-Ad molecule compared to the native sequence, and were able to inhibit activation of an I-Ad-restricted T cell hybridoma in vitro. The induction of a T cell proliferative response to a peptide antigen could be inhibited by co-administration of core-extended peptide with antigen in the same adjuvant emulsion. Furthermore, inhibition also occurred when the inhibitor molecule was delivered separately one day before immunization. Finally, the induction of the autoimmune disease, experimental allergic encephalomyelitis (EAE), in susceptible mice could be reduced by the administration of a core-extended peptide with high affinity for the appropriate class II molecule. These findings have implications for the use of MHC antagonists in the control and treatment of MHC-associated autoimmune conditions in humans.

Peptide competition for antigen presentation

Since each major histocompatibility complex (MHC) molecule can bind many different peptides, it might be predicted that competition for the same MHC-binding site takes place between peptides with unrelated sequences. As Luciano Adorini and Zoltan Nagy report here, this does indeed occur, both in vitro and in vivo. In-vivo competition between peptides for antigen presentation to T lymphocytes is an important influence on the immunodominance of T-cell determinants. In addition, it is possible to modulate T-cell activation by interfering with the binding of antigenic peptides to MHC class II molecules. This could represent a suitable approach to a rational treatment of autoimmune diseases and, possibly, of allograft rejection.

Interactions between proteins, peptides and amino acids. New advances 1986-1989

The paper deals with the recent achievements in the study of the various interactions between proteins, peptides and amino acids. The interactions are classified according to the hydrophilic, hydrophobic or mixed character of the interactive forces. The effect of the interaction on protein (peptide) association, structure and biological activity as well as the role of individual amino acid residues in the hydrophobic and hydrophilic interactions are discussed.

Overexpression of native human beta 2-microglobulin in Escherichia coli and its purification

beta 2-Microglobulin (beta 2M), the small subunit of human leukocyte antigen (HLA) class-I proteins, has been synthesized in Escherichia coli and purified in mg amounts. A beta 2m cDNA clone was fused in-frame behind DNA encoding the signal sequence for the outer membrane protein, OmpA. Three different constructions were made, whose products differed by the insertion of either an extra Ala residue, the hexapeptide AEFLEA [single-letter amino acid (aa) code], or no aa between the OmpA signal sequence and beta 2M-coding sequence. All three protein products were correctly processed by bacterial signal peptidase, as determined by N-terminal sequencing, and all three were secreted as soluble proteins into the periplasmic space. However, the signal sequence of the preprotein with the inserted hexapeptide, AEFLEA, was cleaved to a much greater degree than the other two preproteins. When there was no insertion, the mature protein was identical to human beta 2M, as analyzed by sodium dodecyl sulfate-polyacrylamide-gel electrophoresis, circular dichroism, and native isoelectric focusing. This 'bacterial beta 2M', radiolabeled with Bolton-Hunter reagent, was able to exchange into papain-solubilized HLA-B7, as determined by Sephadex G-75 chromatography and immune precipitation, indicating that bacterial beta 2M could complex with the heavy chain of HLA-B7.