Critical Roles of O-Acetylation Revealed by Synthetic Trisaccharide Antigens for a Broad-Spectrum Anti-Salmonella Vaccine.

Salmonella infections are a major global health burden, exacerbated by rampant antimicrobial resistance and the narrow serovar coverage of current vaccines. To overcome the limitations of current vaccine design, structurally defined trisaccharides derived from Salmonella O-polysaccharide backbones composed of D-mannose-α(1→4)-L-rhamnose-α(1→3)-D-galactose-α(1→2) sequence and differing only in O-acetylation patterns on the rhamnose were synthesized and conjugated with mQβ virus-like particles. The resulting glycoconjugates elicited robust glycan-specific antibody responses. Interestingly, while the vaccine based on the non-acetylated antigen was not protective against typhoidal strains of Salmonella, rabbit antisera induced by the diacetylated rhamnose containing trisaccharide conjugate provided effective protection to mice against lethal challenges by four major pathogenic Salmonella serovars including both typhoidal and non-typhoidal serovars. Mechanistic studies revealed that antigen di-O-acetylation significantly enhanced complement deposition and opsonophagocytosis of bacteria by the antibodies produced. This is the first time that a vaccine based on a single antigen protected against all four major pathogenic serovars to humans, opening new ground for broad spectrum anti-Salmonella vaccine design. In addition, this suggests introduction of defined acetylation to synthetic O-antigen can be a promising strategy for vaccine development as many bacterial O-polysaccharides are modified by acetates in nature.