Evaluation of the Immunogenicity and Biological Activity of the Citrobacter freundii Vi-CRM197 Conjugate as a Vaccine for Salmonella enterica Serovar Typhi

Typhoid fever remains a major health problem in developing countries. Young children are at high risk, and a vaccine effective for this age group is urgently needed. Purified capsular polysaccharide from Salmonella enterica serovar Typhi (Vi) is licensed as a vaccine, providing 50 to 70% protection in individuals older than 5 years. However, this vaccine is ineffective in infants. Vi conjugated to a carrier protein (i.e., an exoprotein A mutant from Pseudomonas aeruginosa [rEPA]) is highly immunogenic, provides long-term protection, and shows more than 90% protective efficacy in children 2 to 5 years old. Here, we describe an alternative glycoconjugate vaccine for S. Typhi, Vi-CRM197, where Vi was obtained from Citrobacter freundii WR7011 and CRM197, the mutant diphtheria toxin protein, was used as the carrier. We investigated the optimization of growth conditions for Vi production from C. freundii WR7011 and the immunogenicity of Vi-CRM197 conjugates in mice. The optimal saccharide/protein ratio of the glycoconjugates was identified for the best antibody production. We also demonstrated the ability of this new vaccine to protect mice against challenge with Vi-positive Salmonella enterica serovar Typhimurium.