Interaction of the receptor binding domains of Pseudomonas aeruginosa pili strains PAK, PAO, KB7 and P1 to a cross-reactive antibody and receptor analog: implications for synthetic vaccine design11Edited by P. E. Wright

The four synthetic peptide antigens, PAK 128-144, PAO 128-144, KB7 128-144 and P1 126-148, correspond in amino acid sequence to the C-terminal receptor binding regions of four strains (PAK, PAO, KB7, P1) of Pseudomonas aeruginosa pilin. The NMR solution structures of the trans forms of the peptides show conserved β-turns which have been implicated in antibody and receptor recognition. The interactions between these peptides and a cross-reactive monoclonal antibody, PAK-13, have been studied using two-dimensional1H NMR spectroscopy in order to map the antigenic determinants recognized by the antibody. Residues for which spectral changes were observed upon antibody binding differed from peptide to peptide but were mostly confined to one or both of the turn regions and to the hydrophobic pockets. Conformational changes in the β-turns and hydrophobic pockets of these peptides upon antibody binding were also monitored by examination of the pattern of nuclear Overhauser effects (NOEs) versus transferred nuclear Overhauser effects (TRNOEs) for the free versus the bound peptides. Although TRNOEs developed strongly between side chain resonances in the hydrophobic pockets of the peptides, no additional backbone TRNOEs were observed in the presence of antibody, suggesting no major conformational changes in the secondary structures of the peptides upon binding. This implies a flexible antibody combining site, a feature which is discussed with respect to cross-reactivity, strain specificity, and the design of a synthetic peptide vaccine effective against a broad spectrum of P. aeruginosa strains. The binding of the PAK peptide to a disaccharide receptor analog, (βGalNAc(1-4)βGal), was also studied using 1H NMR in order to map the “adhesintope” recognized by the receptor. Spectral changes observed in the peptide spectrum with the binding of receptor were similar to those seen for the binding of antibody, suggesting that the epitope recognized by the antibody is structurally coincident with the adhesintope recognized by the receptor. The relevancy of this result is discussed with respect to immunogenicity versus pathogenicity, and the proper design of a vaccine which could prevent the mutational escape of the pathogen away from the host’s defence systems.