Conformational study of a short Pertussis toxin T cell epitope incorporated in a multiple antigen peptide template by CD and two-dimensional NMR. Analysis of the structural effects on the activity of synthetic immunogens.

The immunogenic efficacy of multiple antigen peptides, MAPs, i.e. branched molecules in which multiple copies of a given immunogenic peptide are attached on a scaffold of lysine residues via both alpha and epsilon linkages, has been repeatedly demonstrated, but little is known about the structural arrangement of these peptide constructs. A conformational characterization was therefore performed for a known T cell epitope of the S1 subunit of Pertussis toxin, whose sequence is predicted to form alpha-helix. The peptide DNVLDHLTGR, its N-acetylated and C-amidated analogue and a tetrabranched MAP based on the N-acetylated peptide were prepared and studied by CD and two-dimensional 1H-NMR. No evidence of helical structure was obtained in water for the isolated peptides. In contrast, in triflouroethanol, the isolated epitopes fold into a helical structure spanning the segment Val3-Thr8 in the uncapped molecule and encompassing also the N-terminal region in the capped analogue. The mobile C-terminal region tends to adopt a distorted turn arrangement in both peptides due to the folding of Arg10 guanidinium over the backbone. No distortion of the helix structure was observed for the single-copy epitope in the four-branched MAP molecule in trifluoroethanol: each peptide chain is equivalent within the MAP and shows an even more regular helical pattern than the isolated end-blocked sequence. A slight difference was located at the junction with the lysine scaffold: the peptide bond to epsilon NH was found in a much more extended conformation than the corresponding link to alpha NH. These structural results correlate with in vitro T cell stimulatory activity of the three compounds examined and provide arguments supporting the previous suggestion that MAP tetramers are unlikely to elicit an immune response specific for the synthetic template assembly, a feature necessary to retain the advantage of the polymeric epitope presentation.