The mode of ligand recognition by two peptide : MHC class I-specific monoclonal antibodies

The Ig superfamily members TCR and B cell receptor (BCR) share high structu ral and amino acid homology, yet interact with Ags in a distinct manner. Th e overall shape of the TCR ligand is rather constant, with the variation co ming from the MHC polymorphism and the peptide heterogeneity, Consequently, the TCR alpha- and beta-chains form a relatively flat ligand-binding site that interacts with the peptide:MHC (pep:MHC) ligand in a fixed diagonal or ientation relative to the MHC alpha-helices, with the alpha- and beta-chain s of the TCR contacting the N and C termini of the pep:MHC complex, respect ively. By contrast, the shape of BCR ligands varies dramatically, and the B CR exhibits much greater variability of the Ag-binding site. The mAbs 25-D1 ,16 (D1) and 22-C5.9 (C5), specific for the OVA-8:H-2K(b) complex, allowed us to directly compare how TCR and BCR approach the same ligand, To that ef fect, we mapped D1 and C5 footprints over the OVA-8:H-2K(b) complex. Using peptide variants and mutant MHC molecules, we show that the D1 and C5 conta cts with the OVA-8:K-b complex C terminus overlap with the TCR beta-chain f ootprint, but that this footprint also extends to the regions of the molecu le not contacted by the TCR, These studies suggest that D1 and C5 exhibit a hybrid mode of pep:MHC recognition, in part similar to that of the TCR bet a-chain and in part similar to the conventional anti-MHC Ab.