Design of Glycoconjugate Vaccines against Invasive African Salmonella enterica Serovar Typhimurium
Rondini S., Micoli F., Lanzilao L., Gavini M., Alfini R., Brandt C., Clare S., Mastroeni P., Saul A., MacLennan CA.
Nontyphoidal salmonellae, particularlySalmonella entericaserovar Typhimurium, are a major cause of invasive disease in Africa, affecting mainly young children and HIV-infected individuals. Glycoconjugate vaccines provide a safe and reliable strategy against invasive polysaccharide-encapsulated pathogens, and lipopolysaccharide (LPS) is a target of protective immune responses. With the aim of designing an effective vaccine againstS. Typhimurium, we have synthesized different glycoconjugates, by linking O-antigen and core sugars (OAg) of LPS to the nontoxic mutant of diphtheria toxin (CRM197). The OAg-CRM197conjugates varied in (i) OAg source, with threeS. Typhimurium strains used for OAg extraction, producing OAg with differences in structural specificities, (ii) OAg chain length, and (iii) OAg/CRM197ratio. All glycoconjugates were compared for immunogenicity and ability to induce serum bactericidal activity in mice.In vivoenhancement of bacterial clearance was assessed for a selectedS. Typhimurium glycoconjugate by challenge with liveSalmonella. We found that the largest anti-OAg antibody responses were elicited by (i) vaccines synthesized from OAg with the highest glucosylation levels, (ii) OAg composed of mixed- or medium-molecular-weight populations, and (iii) a lower OAg/CRM197ratio. In addition, we found that bactericidal activity can be influenced byS. Typhimurium OAg strain, most likely as a result of differences in OAg O-acetylation and glucosylation. Finally, we confirmed that mice immunized with the selected OAg-conjugate were protected againstS. Typhimurium colonization of the spleen and liver. In conclusion, our findings indicate that differences in the design of OAg-based glycoconjugate vaccines against invasive AfricanS. Typhimurium can have profound effects on immunogenicity and therefore optimal vaccine design requires careful consideration.