Pilin-based anti-Pseudomonas vaccines: latest developments and perspectives.

Among the several adhesins produced by Pseudomonas aeruginosa (Pa), the type-4 pilus promotes the majority of the adherence capability of the bacterium to epithelial cells and it is a major virulence factor in an AB.Y/SnJ mouse infection model. Vaccines targeting the disulfide loop (DSL) adherence binding domain of the pilin protein should therefore provide an effective protection against initial colonization and infection with Pa. To selectively elicit adherence blocking antibodies, the pilin DSL domain was chosen as peptide antigen for the construction of recombinant protein and live vaccines. While synthetic peptide-carrier protein conjugates provided some strain-specific protection, chimeric proteins with N- or C-terminally fused pilin DSL peptides did not engender protective IgG titers mice. Integral fusions of the pilin DSL peptide with the minor coat protein of filamentous phage or surface exposed regions of an outer membrane protein resulted in a display of the peptide on the surface of the phage particles and bacterial cells respectively. However, in immunization studies neither of these live vaccines were effective immunogens. The paracrystalline S-layer of Caulobacter crescentus combines several advantages of an effective antigen surface display system. Recombinant S-layer proteins with singlecopy insertions of a pilin peptide did not engender significant IgG titers, whereas multiple tandem insertions of the same peptide increased the serum IgG response in mice a thousand times. Multiple insertions of DSL peptides from different frequent pilin prototypes may be an interesting alternative for a recombinant cross-protective anti-Pseudomonas vaccine.