Evidence for cobinding of self- and allopeptides to human class II major histocompatibility antigen DR1 by energy transfer

HLA-DM mediates epitope selection by a "compare-exchange" mechanism when a potential peptide pool is available

Background

HLA-DM (DM) mediates exchange of peptides bound to MHC class II (MHCII) during the epitope selection process. Although DM has been shown to have two activities, peptide release and MHC class II refolding, a clear characterization of the mechanism by which DM facilitates peptide exchange has remained elusive.

Methodology/Principal Findings

We have previously demonstrated that peptide binding to and dissociation from MHCII in the absence of DM are cooperative processes, likely related to conformational changes in the peptide-MHCII complex. Here we show that DM promotes peptide release by a non-cooperative process, whereas it enhances cooperative folding of the exchange peptide. Through electron paramagnetic resonance (EPR) and fluorescence polarization (FP) we show that DM releases prebound peptide very poorly in the absence of a candidate peptide for the exchange process. The affinity and concentration of the candidate peptide are also important for the release of the prebound peptide. Increased fluorescence energy transfer between the prebound and exchange peptides in the presence of DM is evidence for a tetramolecular complex which resolves in favor of the peptide that has superior folding properties.

Conclusion/Significance

This study shows that both the peptide releasing activity on loaded MHCII and the facilitating of MHCII binding by a candidate exchange peptide are integral to DM mediated epitope selection. The exchange process is initiated only in the presence of candidate peptides, avoiding possible release of a prebound peptide and loss of a potential epitope. In a tetramolecular transitional complex, the candidate peptides are checked for their ability to replace the pre-bound peptide with a geometry that allows the rebinding of the original peptide. Thus, DM promotes a “compare-exchange” sorting algorithm on an available peptide pool. Such a “third party”-mediated mechanism may be generally applicable for diverse ligand recognition in other biological systems.

A dipalmitoyl peptide that binds SH3 domain, disturbs intracellular signal transduction, and inhibits tumor growth in vivo

The Src homology 3 (SH3) domain plays a crucial role in protein-protein interactions during intracellular signal transduction. Blocking the SH3-mediated protein binding may inhibit the corresponding signal transduction, and thus, block the cellular functions. In this study, a peptide that specifically binds to SH3 domain could be introduced into the intracellular region when the peptides were conjugated with dipalmitic acid and appeared to disturb intracellular signaling. The dipalmitoyl peptide appeared to inhibit the phosphorylation of ZAP-70, Lck, and T-cell antigen receptor zeta in Jurkat. Mobilization of the intracellular free calcium induced by anti-CD3 antibody was reduced after treatment with the dipalmitoyl peptide. It was also observed that the dipalmitoyl peptide inhibited cancer cell growth both in vitro and in vivo. These results suggest that the dipalmitoyl peptide that presumably disturbs SH3-mediated signal transduction may have a potent anti-proliferative activity, which would be useful as a potential anti-tumor agent.

HLA-B7 beta-pleated sheet-derived synthetic peptides are immunodominant T-cell epitopes regulating alloresponses

Chronic rejection of transplanted allografts is the major cause of graft loss after clinical solid organ transplantation. Recent data link the indirect presentation of allopeptides to chronic graft loss; thus, identification of immunodominant epitopes on major histocompatibility complex (MHC) antigens could significantly contribute to establishing novel ways for monitoring and managing chronic rejection. Here, we show that synthetic allo-MHC-derived peptides covering the polymorphic region 56 to 120 of HLA-B7 modulate alloresponses. In particular, the 2 beta-pleated sheet-derived peptides covering residues 91 to 105 and 96 to 120, respectively, but not sequences from the alpha1 helix, were presented by autologous peripheral blood lymphocytes to induce T-cell proliferation. In addition, the 2 beta-pleated sheet-derived peptides and the alpha1-derived peptide residues 60 to 75 abrogated lysis of HLA-B7 target cells by anti-HLA-B7 cytotoxic T lymphocytes (CTLs). Although most residues between 91 and 120 are normally not directly accessible to T cells, our results indicate that peptides derived from the lower surface of the peptide-binding groove of HLA-B7 are immunodominant in HLA-B7 alloresponses. To characterize the binding and stability of allopeptides to T cells, the 62-70 peptide-derived from the 60-75 allopeptide that blocked cytotoxicity of anti-HLA-B7 CTL-was synthesized and coupled with fluorescein isothiocyanate. The peptide specifically labeled anti-B7 CTL, but not anti-HLA-A2 CTL as measured by flow cytometry. Peptide binding to CTL was specific at 4 degrees C and remained stable for 12 hours, whereas it remained stable for less than 2 hours at 37 degrees C. These studies allow the identification of HLA-B7 T-cell epitopes and reveal for the first time a novel, previously unrecognized application of synthetic HLA-derived allopeptides to visualize alloreactive T cells.

The tetramer model: a new view of class II MHC molecules in antigenic presentation to T cells

Crystallographic studies suggest a plausible divalent interaction between T-cell receptor (TCR) and MHC class II molecules. In addition, biochemical data suggest that these divalent MHC molecules are preformed at the membrane of the antigen-presenting cell. The tetramer model is based on these preformed tetrameric class II molecules that can be loaded with identical or different peptides in their two grooves. This enables divalent class II molecules to deliver two different messages to T cell: 1) a two-peptide message, in which the tetramer with two identical peptides is able to cross-link two TCRs triggering full activation of a T cell. At the thymic level we propose that this message induces negative selection; or 2) a one-peptide message: only one of the peptides loaded in the class II tetramer is able to interact with that TCR. This message would be involved in triggering partial activation phenomena in mature lymphocytes, whereas in thymocytes this message would mediate positive selection. Since high concentrations of a peptide would favor the load of tetramers with identical peptides, the tetramer could therefore be viewed as a quantitative-qualitative transducer that would trigger different responses depending on the concentration of antigenic peptides.

Determination of HLA-B7 T-cell epitopes

A series of peptides derived from the alpha 1 and alpha 2 regions of the HLA-B7 and HLA-A2 molecules was synthesized with an automatic peptide synthesizer using the FMOC (9-fluorenylmethoxycarbonyl) technique. Peptides were analyzed and purified by reversed-phase high pressure liquid chromatography (HPLC). Peptide purity was > 96%. The effect of B7 peptides on mixed lymphocyte cultures was tested in vivo. The alloresponse in the cultures with B7 peptides was strongly enhanced. The peptides that were most effective inhibited cytotoxic T-lymphocyte (CTL) cytolysis. The data show that the peptides are immunogenic and that they are recognised by both the direct and indirect pathways. Further, the mechanism of peptide recognition was studied. We coupled peptide B7 (residues 62-70 in HLA-B7) and peptide A2 (residues 62-70 in HLA-B2) covalently to fluorescein isothiocyanate (FITC). B7-specific CTLs were incubated with these peptides at 37 degrees C for 90 min and cell fluorescence measured by flow cytometry. The B7 peptide was bound by 60% of the CTLs whereas the A2 peptide, as a negative control, bound only 10%. The molecular size of the ligands to which the peptides bind are being characterized by immunoprecipitation.

Self-release of CLIP in peptide loading of HLA-DR molecules

The assembly and transport of major histocompatibility complex (MHC) class II molecules require interaction with the invariant chain. A fragment of the invariant chain, CLIP, occupies the peptide-binding groove of the class II molecule. At endosomal pH, the binding of CLIP to human MHC class II HLA-DR molecules was counteracted by its amino-terminal segment (residues 81 to 89), which facilitated rapid release. The CLIP (81-89) fragment also catalyzed the release of CLIP(90-105) and a subset of other self-peptides, probably by transient interaction with an effector site outside the groove. Thus, CLIP may facilitate peptide loading through an allosteric release mechanism.