UNUSUAL TOPOLOGY OF AN HLA-B27 ALLOSPECIFIC T-CELL EPITOPE LACKING PEPTIDE SPECIFICITY

Recognition of MHC + peptide complexes by TCRs is thought to involve a large surface formed by exposed residues from the bound peptide and f rom the a-helices of the MHC protein. This interaction appears to be e ssentially symmetrical in the positioning of the TCR relative to the M HC molecule. In this study the topology of HLA-B27 recognition by an a lloreactive TCR, 64.8P, has been analyzed with a panel of site-specifi c mutants that have changes at multiple positions along the peptide bi nding site of HLA-B27. Abrogation of transfectant target eel I lysis b y CTL 64.8P was obtained only with some mutations in the peptide side chain binding pockets A and B, whereas little or no effect was observe d with mutations outside these pockets. CTL 64.8P efficiently lysed mu rine transfectant cells, including HLA-B27(+) RMA-S cells. Recognition of this latter transfectant was more efficient upon increased HLA-B27 expression at 260 degrees C. The uneven distribution of mutations aff ecting HLA-B27 allorecognition by CTL 64.8P strongly suggests an asymm etrical topology in the interaction of this TCR with HLA-B27, in which most of the binding energy is provided by contacts with HLA-B27 and/o r peptide residues located close to pockets A and B, with little contr ibution from other areas of the MHC or peptide molecules. Its conserva tion in RMA-S cells further suggests that the epitope recognized by CT L 64.8P is either peptide-independent or requires any of a set of pept ides having the same amino-terminal residues.