Photoaffinity labeling demonstrates binding between Ia molecules and nominal antigen on antigen-presenting cells

Photoaffinity antigens for human gammadelta T cells

Vgamma2Vdelta2 T cells comprise the major subset of peripheral blood gammadelta T cells in humans and expand during infections by recognizing small nonpeptide prenyl pyrophosphates. These molecules include (E)-4-hydroxy-3-methyl-but-2-enyl-pyrophosphate (HMBPP), a microbial isoprenoid intermediate, and isopentenyl pyrophosphate, an endogenous isoprenoid intermediate. Recognition of these nonpeptide Ags is mediated by the Vgamma2Vdelta2 T cell Ag receptor. Several findings suggest that prenyl pyrophosphates are presented by an Ag-presenting molecule: contact between T cells and APC is required, the Ags do not bind the Vgamma2Vdelta2 TCR directly, and Ag recognition is abrogated by TCR mutations in CDRs distant from the putative Ag recognition site. Identification of the putative Ag-presenting molecule, however, has been hindered by the inability to achieve stable association of nonpeptide prenyl pyrophosphate Ags with the presenting molecule. In this study, we show that photoaffinity analogues of HMBPP, meta/para-benzophenone-(methylene)-prenyl pyrophosphates (m/p-BZ-(C)-C(5)-OPP), can crosslink to the surface of tumor cell lines and be presented as Ags to gammadelta T cells. Mutant tumor cell lines lacking MHC class I, MHC class II, beta(2)-microglobulin, and CD1, as well as tumor cell lines from a variety of tissues and individuals, will all crosslink to and present m-BZ-C(5)-OPP. Finally, pulsing of BZ-(C)-C(5)-OPP is inhibited by isopentenyl pyrophosphate and an inactive analog, suggesting that they bind to the same molecule. Taken together, these results suggest that nonpeptide Ags are presented by a novel-Ag-presenting molecule that is widely distributed and nonpolymorphic, but not classical MHC class I, MHC class II, or CD1.

Signalling mechanisms of endothelin-induced mitogenesis and melanogenesis in human melanocytes

To understand the signalling mechanisms involved in the dual stimulatory effects of endothelin-1 (ET-1) on DNA synthesis and melanization in cultured human melanocytes, we analysed the biological profile of ET-1 receptor and determined the effects of ET-1 on the protein kinase C, cyclic AMP system and mitogen-activated protein kinase (MAP kinase) in comparison with their relevant stimulants. The photoaffinity labelling of ET-1 receptors with Denny-Jaff reagents revealed an ET-1 receptor with a molecular mass of 51 kDa in human melanocytes. The ET(A) receptor subtype-sensitive antagonist BQ123(50 nM) or pertussis toxin (100 ng/ml) significantly suppressed the ET-1-induced intracellular calcium mobilization, indicating the presence of pertussis toxin-sensitive G-protein-coupled ET(A) receptors. An assay of protein kinase C activity revealed that 10nM ET-1 translocated cytosolic protein kinase C to membrane-bound protein kinase C within 5 min of the start of incubation. In contrast, receptor-mediated melanocyte activation by ET-1 was accompanied by an elevated level of cyclic AMP (4-fold over control) after 10-60 min of incubation, whereas 60 min of incubation of human melanocytes with c-Kit or c-Met ligands such as stem cell factor (10 nM) or basic fibroblast growth factor (10 nM) did not elevate the cyclic AMP level. We have also demonstrated that a specific tyrosine kinase inhibitor, tyrphostin B-42 (10 microM), inhibited the ET-1-induced growth stimulation, suggesting the involvement of the tyrosine kinase pathway in growth stimulation. Consistently, an assay of MAP kinase revealed that ET-1 caused a 10-fold activation of MAP kinase after 5 min of incubation with human melanocytes in a similar way to tyrosine kinase ligands such as stem cell factor and hepatocyte growth factor. Further, the DNA synthesis stimulated by the c-Kit ligand stem cell factor at a concentration of 1 nM was synergistically enhanced by 5 nM ET-1. These results suggest that ET-induced dual cellular events in human melanocytes are closely associated with cross-talk between the protein kinase C and A and tyrosine kinase pathways.

Reduction of disulfide bonds during antigen processing: evidence from a thiol-dependent insulin determinant

Previous studies have demonstrated that insulin, like other protein antigens, requires processing in metabolically active antigen-presenting cells (APC) before it can be recognized by class II-restricted T lymphocytes. Unlike many other proteins, insulin peptides of minimal size retain the requirement for antigen processing. We demonstrate that this requirement can be bypassed by incubation of insulin with reducing agents in the presence of aldehyde-fixed APC. Fixed APC treated in this way were able to stimulate I-Ab- and I-Ad-restricted T cell hybridomas. Data are presented that demonstrate that cloned and polyclonal T cells recognize a determinant within the NH2-terminal 14 residues of the beef insulin A chain with no requirement for B chain residues. The common feature among peptides capable of stimulating these cells in the presence of live APC is the chemical form of the cysteine thiol groups. Those forms that produce free thiols upon reduction are active, whereas those with irreversibly protected sulfhydryls are not. Functional experiments with fixed APC and competition binding experiments with purified I-Ad indicate that only A chain peptides with free thiols are able to stably associate with the peptide-binding site on class II in a form that is recognized by specific T cells. Our findings indicate that reduction of disulfide bonds is both necessary and sufficient for presentation of insulin to a major population of class II-restricted T cells. The results provide strong support for the hypothesis that protein disulfides can be reduced during physiologic antigen processing.

The relationship between antigen concentration, antigen internalization, and antigenic complexes: modeling insights into antigen processing and presentation

Native antigen is processed and subsequently presented on the surface of antigen-presenting cells, an important step in the elicitation of an immune response. The early events of antigen processing and presentation include: ingestion of a native antigen, intracellular degradation to expose an antigenic peptide fragment, binding of this fragment with an MHC class II molecule, and display of this newly formed complex on the cell surface. Through the development of a mathematical model, a set of mathematical equations which describes the time-dependent appearance, disappearance, and movement of individual molecules, quantitative insight can be gained into the pathways and rate-limiting steps of antigen presentation. The credibility of the model has been verified by comparison to literature data. For example, it has been shown experimentally that macrophages require 60 min for effective antigen presentation, whereas B cells require 6-8 h. The mathematical model predicts these presentation times and identifies the difference in the cell's respective pinocytic rates and sizes as important parameters. B cells capture antigen in their environment through nonspecific fluid-phase pinocytosis as well as by binding antigen to their surface immunoglobulin, allowing receptor-mediated uptake. Uptake of antigen via receptor-mediated endocytosis has been reported to require 1,000-fold less antigen than uptake via nonspecific pinocytosis. The mathematical model clearly predicts this decrease in concentration. The model also makes quantitative predictions for the number of MHC class II-antigen complexes needed to produce T cell stimulation.

Tolerance in transgenic mice expressing major histocompatibility molecules extrathymically on pancreatic cells

Transgenic mice with defined expression of major histocompatibility complex (MHC) proteins provide novel systems for understanding the fundamental question of T cell tolerance to nonlymphoid self components. The MHC class II I-E and I-A and class I H-2K molecules expressed specifically on pancreatic islet or acinar cells serve as model self antigens. In these systems, transgenic proteins are not detected in the thymus or other lymphoid tissues. Yet mice are tolerant to the pancreatic MHC products in vivo; this tolerance is not induced by clonal deletion. These studies have been aided by monoclonal antibodies specific for I-E-reactive T cells and indicate that clonal anergy may be an important mechanism of tolerance to peripheral 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.

Insulin autoantibody polymorphisms with greater discrimination for diabetes in humans

Insulin autoantibodies, like islet cell antibodies, are found not only in the sera of newly diagnosed Type 1 (insulin-dependent) diabetic patients and their relatives, but also in patients with other autoimmunities who do not develop diabetes. Insulin autoantibodies are oligo/monoclonal and frequently binding-site restricted. As determinant selection is genetically determined, we questioned whether certain polymorphisms of insulin autoantibodies, identified by their binding site on the insulin molecule, could better discriminate for Type 1 diabetes, which is also HLA determined. First, we raised monoclonal antibodies to human insulin by classic fusion methods in order to determine the range of antibody polymorphism, and identified five distinct types by their binding profiles to a panel of insulin variants, using an enzyme-linked immunosorbent assay. Two of these polymorphisms, type A and type B, were subsequently found in insulin autoantibody positive human sera using the same panel of insulin variants, and successfully distinguished diabetes-related from diabetes-unrelated individuals. Thus, the type B polymorphism was responsible for binding in 60% of 41 insulin autoantibody positive individuals with polyautoimmune disease but no personal or family history of diabetes (diabetes unrelated), but in only 2% of a group which comprised 17 newly-diagnosed insulin autoantibody positive Type 1 diabetic patients, 19 insulin autoantibody positive discordant twins of Type 1 diabetes and six insulin autoantibody positive healthy siblings of Type 1 diabetic patients (diabetes related) (p less than 0.01). Isolation of the type A polymorphism alone reduced the proportion of false negatives in the insulin autoantibody test for diabetes relatedness from 49% to 20% without diminishing its specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of photoreactive lysozyme peptides to murine histocompatibility class II molecules

We have studied the interaction of six photoreactive conjugates of the immunogenic hen egg-white lysozyme peptides HEL(46-61) or HEL(49-61) with the murine histocompatibility class II molecules I-Ak, I-Ad, I-Ek, and I-Ed. All compounds tested selectively labeled the alpha chain of the class II molecules. This was true when testing class II molecules on cell membranes or solubilized in detergents. The COOH-terminal conjugate of HEL(49-61) with (4-azidobenzoyl)cystine preferentially labeled I-Ak. However, addition of hydroxyl or iodine substituents to the photoreactive moiety increased the labeling efficiency and resulted in labeling of the other class II molecules. The data suggest that the photoreactive groups enhanced the binding affinities of these peptides to class II molecules, reflected by the increased labeling efficiencies. Conversely, introduction of an iodine substitution into the tyrosine residue of HEL(46-61) or HEL(49-61) strongly decreased the photoaffinity labeling, possibly due to steric interference with ligand binding to class II molecules. Judicious use of photoaffinity probes that conserve binding specificity of the peptide should be useful for mapping the antigen-binding site of a class II molecule.

Overlapping T cell antigenic sites on a synthetic peptide fragment from herpes simplex virus glycoprotein D, the degenerate MHC restriction elicited, and functional evidence for antigen-Ia interaction

Analysis of the B10.A T cell response to synthetic peptides representing the NH2-terminal 23 amino acids from the HSV glycoprotein D sequence revealed two antigenic determinants for T cells: one localized between residues 1-16 and the other between residues 8-23. The 1-16 site, which is helical, was recognized in the context of the Ia molecule, whereas the 8-23 site, which is nonhelical, was recognized in the context of the I-E molecule. The I-E-restricted response was found to be highly MHC degenerate in that T cell hybridomas specific for the 8-23 peptide responded to antigen on APCs derived from B10.A, B10.A(5R), and B10.A(9R) mice and showed differences in antigenic fine specificity with APCs of different haplotypes. These data support the idea of antigen-Ia interaction.

Identification of a peptide binding protein that plays a role in antigen presentation

The helper T-cell response to globular proteins appears, in general, to require intracellular processing of the antigen, such that a peptide fragment containing the T-cell antigenic determinant is released and transported to and held on the surface of an Ia-expressing, antigen-presenting cell. However, the molecular details underlying these phenomena are largely unknown. The means by which antigenic peptides are anchored on the antigen-presenting cell surface was investigated. A cell surface protein is identified that was isolated by its ability to bind to a 24-amino acid peptide fragment of pigeon cytochrome c, residues 81-104, containing the major antigenic determinant for B10.A mouse T cells. This peptide binding protein, purified from [35S]methionine-labeled cells, appears as two discrete bands of approximately equal to 72 and 74 kDa after NaDodSO4/PAGE. The protein can be eluted from the peptide affinity column with equivalent concentrations of either the antigenic pigeon cytochrome c peptide or the corresponding nonantigenic peptide of mouse cytochrome c. However, it does not bind to the native cytochromes c, either of pigeon or mouse, and thus the protein appears to recognize some structure available only in the free peptides. This protein plays a role in antigen presentation as evidenced by the ability of rabbit antibodies raised against it to block the activation of an antigen-specific T-cell hybrid by antigen-presenting cells and pigeon cytochrome c. Its expression is not major histocompatibility complex-restricted in that the blocking activity of the antisera can be absorbed on spleen cells from mice of different haplotypes. This peptide binding protein can be isolated from a variety of cell types, including B cells, T cells, and fibroblasts. The anchoring of processed peptides on the cell surface by such a protein may play a role in antigen presentation--facilitating the interaction of antigenic peptides with Ia and/or the T-cell receptor.

High-affinity fluorescent peptide binding to I-Ad in lipid membranes

Membranes isolated from antigen-pulsed, antigen-presenting cells were extensively dialyzed and shown to retain the ability to present antigen to an I-Ad-restricted, antigen-specific T cell, 3DO-54.8. This ability to retain antigen on the membrane was duplicated in lysates of antigen-presenting cells as well as with pure I-A molecules in phospholipid vesicles. Measurement of the concentration of surface-associated fluorescent peptide on planar membranes prepared from antigen-pulsed phospholipid vesicles showed that about 1 peptide molecule was retained per 100 I-A molecules. We calculate that about 1000 I-A-associated peptide molecules are sufficient to stimulate the response of a single 3DO-54.8 cell. The association of fluorescent peptide with the surface depended on the presence of I-A and was blocked by unlabeled ovalbumin peptide or by a digest of hen egg lysozyme, added at the time of the pulse, but not after pulsing.