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Characterization of CD8+ Effector T Cell Responses in Volunteers Immunized with Salmonella enterica Serovar Typhi Strain Ty21a Typhoid Vaccine1

Salmonella enterica serovar Typhi (S. typhi) strain Ty21a remains the only licensed attenuated typhoid vaccine. Despite years of research, the identity of the protective immunological mechanisms elicited by immunization with the Ty21a typhoid vaccine remains elusive. The present study was designed to characterize effector T cell responses in volunteers immunized with S. typhi strain Ty21a typhoid vaccine. We determined whether immunization with Ty21a induced specific CTL able to lyse S. typhi-infected cells and secrete IFN-γ, a key effector molecule against intracellular pathogens. We measured the functional activity of these CTL by a 51Cr-release assay using 8-day restimulated PBMC from Ty21a vaccinees as effector cells and S. Typhi-infected autologous PHA-activated PBMC as target cells. Most vaccinees exhibited consistently increased CD8-mediated lysis of targets by postimmunization PBMC when compared with preimmunization levels. We also developed an IFN-γ ELISPOT assay to quantify the frequency of IFN-γ spot-forming cells (SFC) in PBMC from Ty21a vaccinees using an ex vivo system. Significant increases in the frequency of IFN-γ SFC following immunization (mean ± SD, 393 ± 172; range 185–548 SFC/106 PBMC; p = 0.010), as compared with preimmunization levels, were observed. IFN-γ was secreted predominantly by CD8+ T cells. A strong correlation was recorded between the cytolytic activity of CTL lines and the frequency of IFN-γ SFC (r2 = 0.910, p < 0.001). In conclusion, this work constitutes the first evidence that immunization of volunteers with Ty21a elicits specific CD8+ CTL and provides an estimate of the frequency of CD8+ IFN-γ-secreting cells induced by vaccination.