2.1.4. S. typhi Clinical Trial
Due to the lack of an animal model, progress to develop safe S. typhi vaccines for human use is slow. Currently, a clinical trial is still the best measurement of safety and effectiveness of S. typhi vaccines or vaccine vectors. Our intensive work carried out in mice lead to the development of a S. typhimurium strain χ9558 with a balance between safety and immunogenicity in adult, neonatal and infant mice . Based on the results, we constructed three recombinant attenuated S. typhi vaccine vectors, χ9633, χ9639, χ9640, with essentially the same genotype as χ9558 carrying plasmid pYA4088 encoding the α-helical fragment of PspA Rx1 (aa 3–285) , but with an additional mutation eliminating the immunosuppressive capsular Vi antigen . The vectors were constructed to test the hypothesis that the immunogenicity of live Salmonella vaccines is, at least in part, on its RpoS status. All three S. typhi vaccine strains are similar to the licensed live attenuated typhoid vaccine Ty21a in their abilities to survive in human blood and human monocytes. They are more sensitive to complement and less able to survive and persist in sewage and surface water than their wild-type counterparts . Adult, infant and neonatal mice immunized with these vectors develop immune responses against PspA and Salmonella antigens. The percentages of protection against S. pneumoniae challenge in adult mice immunized with these vectors are between 50 and 81.3%