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 to120 of HLA-B7 modulate alloresponses. In particular, the 2 -pleated sheet-derived peptides covering residues 91 to 105 and 96 to 120, respectively, but not sequences from the ␣1 helix, were presented by autologous peripheral blood lymphocytes to induce T-cell proliferation. In addition, the 2 -pleated sheetderived peptides and the ␣1-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 peptidebinding groove of HLA-B7 are immunodominant in HLA-B7 alloresponses. To characterize the binding and stability of allopeptides to T cells, the 62-70 peptidederived 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°C and remained stable for 12 hours, whereas it remained stable for less than 2 hours at 37°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.
IntroductionStructural studies show that T-cell receptor (TCR) contacts bind peptide and large stretches of ␣1 and ␣2 major histocompatibility complex (MHC) helices. 1,2 In general, endogenous peptides derived from allo-MHC are seen by T cells as foreign-eliciting alloresponses. 3 T cells may recognize peptide-induced conformational changes, and they may also recognize surface MHC structures in addition to or independent of bound peptide. 4 Furthermore, allospecific cytotoxic T lymphocytes (CTLs) that are peptidedependent and peptide-specific have been described in the literature. 5 In general, however, when donor and recipient MHCs are structurally similar, the endogenous peptide appears to be the dominant target of alloreactive T cells, 5-7 a phenomenon backed by clinical data in graft-versus-host disease. 8 Synthetic allopeptides have been used in many different models to modulate alloresponses and to study alloreactivity. [9][10][11] In other studies, class I-and class II-derived allopeptides were used to induce tolerance. 12-14 By studying a series of rat-derived synthetic allo-MHC peptides, immunodominant regions of allogeneic MHC molecules that modulate graft surviv...
