Specific and cross-reactive immune response to oral Salmonella Typhi Ty21a and parenteral Vi capsular polysaccharide typhoid vaccines administered concomitantly

AbstractBackgroundSince protective efficacy of the current typhoid vaccines—oral whole-cell Salmonella Typhi Ty21a and parenteral Vi-capsular polysaccharide preparation—is not optimal, and no vaccines are available against paratyphoid or non-typhoidal Salmonella (NTS) serotypes, new approaches deserve to be explored. The immunological mechanisms elicited by the two typhoid vaccines are mainly targeted against different structures. We studied whether these vaccines would enhance S. Typhi-specific immune response and cross-reactivity against other Salmonellae, if administered concomitantly.Materials and methodsVolunteers were immunized simultaneously with Ty21a and Vi vaccines (Ty21a+Vi group) or with either of the two singly (Ty21a and Vi groups). All volunteers were investigated for circulating specific and cross-reactive plasmablasts, identified by ELISPOT as IgA, IgG or IgM antibody-secreting cells (ASC) reactive with S. Typhi, S. Paratyphi A/B/C, or selected NTS serotypes (S. Enteritidis, S. Typhimurium).ResultsIn the Ty21a+Vi group, no specific or cross-reactive plasmablasts were detected before vaccination. After vaccination, the number of S. Typhi-specific plasmablasts (878 ASC/106 PBMC, 95%CI 554–1201) proved higher than in the Ty21a (339 ASC/106 PBMC; p<0.001) and Vi (149 ASC/106 PBMC; p<0.001) groups. Likewise, cross-reactive responses in the Ty21a+Vi group were higher than in the Ty21a and Vi groups (Ty21a+Vi vs Ty21a: ASC against S. Paratyphi A/B, S. Enteritidis and S. Typhimurium p<0.05, against S. Paratyphi C p<0.01; Ty21a+Vi vs Vi: against S. Paratyphi C not significant, others p