Specific and Cross-reactive Plasmablast Response in Humans after Primary and Secondary Immunization with Vi Capsular Polysaccharide Typhoid Vaccine.

Secondary immunization with polysaccharide vaccines may imply a risk of hyporesponsiveness. Despite the wide use of typhoid Vi capsular polysaccharide vaccine, its potential tendency to hyporesponsiveness has been inadequately addressed. While previous studies have explored serum antibody responses, we applied a more sensitive approach, a single-cell assay for circulating plasmablasts, to compare primary and secondary responses. Twelve subjects received primary and booster doses of the Vi vaccine (Typherix® ) at 30- to 37-month intervals. Plasmablasts specific to the Vi or typhoidal O antigens or cross-reactive with paratyphoid and non-typhoidal Salmonella strains were identified as antibody-secreting cells (ASC) with ELISPOT. Before vaccinations, none had plasmablasts specific to the antigens tested. Twelve of 12 subjects showed a Vi-specific response after primary, but only eight of 12 after booster vaccination. All responded to typhoidal O-9,12 antigen after both immunizations. The geometric mean of plasmablasts specific to the Vi antigen was 59 (95% CI 24-119) and 1 (0-54) IgA + IgG + IgM-ASC/106 peripheral blood mononuclear cell (PBMC) after primary and booster immunizations, respectively, and 20 (9-49) and 56 (29-103) to the O-9,12 antigen. We detected 1 (0-28) and 17 (6-36) ASC/106 PBMC cross-reactive with Salmonella Paratyphi A; 3 (0-30) and 22 (8-48) with S. Paratyphi B; 3 (0-29) and 18 (7-47) with S. Paratyphi C; 19 (10-34) and 51 (26-94) with Salmonella Enteritidis; and 1 (0-35) and 23 (9-52) with Salmonella Typhimurium, respectively. One-third of the vaccinees, although responding to the O-9,12 antigen, failed to respond to the Vi antigen after booster immunization, suggesting hyporesponsiveness in part of the vaccinees. The findings warrant further investigation.