Evaluation of the Immunogenicity and Biological Activity of the Citrobacter freundii Vi-CRM 197 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 fromSalmonella 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 fromPseudomonas 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 forS . Typhi, Vi-CRM197 , where Vi was obtained fromCitrobacter freundii WR7011 and CRM197 , the mutant diphtheria toxin protein, was used as the carrier. We investigated the optimization of growth conditions for Vi production fromC. 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-positiveSalmonella enterica serovar Typhimurium.