Epitope Mapping of Monoclonal Antibodies using Synthetic Oligosaccharides Uncovers Novel Aspects of Immune Recognition of the Psl Exopolysaccharide of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic Gram-negative bacterium that can cause life-threatening infections in critically ill and cystic fibrosis patients. The Psl exopolysaccharide of P.aeruginosa offers an attractive serotype-independent antigen for the development of immunotherapies. Here, the first chemical synthesis of a panel of oligosaccharides derived from the exopolysaccharide of P.aeruginosa by a synthetic strategy that efficiently deals with the stereoselective installation of several -mannosides and the formation of a mannoside that is extended by saccharide moieties at C-1, C-2, and C-3 in a crowded 1,2,3-cis configuration is described. The approach was employed to prepare tetra-, penta-, and hexa- and decasaccharide part structures. The compounds were employed to define the epitope requirements of several functionally active monoclonal antibodies (mAbs) that can bind three distinct epitopes of Psl (classI, II, and III). The classII mAb reacted potently with each oligosaccharide indicating its epitope resides within the tetrasaccharide and does not require the branched mannoside of Psl. The classIII antibody did not bind the tetra- or pentasaccharide; however, it did react potently with the hexasaccharide and weakly with the decasaccharide, suggesting a terminal glucoside is required for optimal binding. Unexpectedly, the classI mAb did not bind any of the oligosaccharides indicating that Psl contains a yet to be elucidated sub-stoichiometric isoform. This study demonstrates that functional activity of a mAb does not only depend on the avidity of binding but also on the location of an epitope within a bacterial polysaccharide. The results also provide a strong impetus to analyze further the structure of Psl to identify the classI epitope, that is expected to provide an attractive target for the development of a synthetic vaccine for P.aeruginosa.