T cells are likely to play an important role in the host defense against Salmonella enterica serovar Typhi, the causative agent of
typhoid fever. We have shown that HLA-E can function as a restriction
element for S. Typhi-specific CD8(+) T cells. Because of the potential importance of HLA-E-restricted CD8(+) responses in resistance to
Salmonella infection, we characterized these responses and investigated their kinetics of appearance and persistence in volunteers immunized orally with the licensed attenuated Ty21a strain
typhoid vaccine. Cells were obtained from volunteers before and at days 2, 4, 7, 10, 14, 28, 42, 56, 120, 180, 360, and 720 after immunization. An ex vivo multicolor staining panel including
antibodies to CD107a and -b,
interleukin-2,
gamma interferon (IFN-gamma), and
tumor necrosis factor alpha (
TNF-alpha) was used to functionally assess memory T-cell subsets by flow cytometry. Increases in
cytokine-secreting CD8(+) cells were observed in the T effector/memory (T(EM)) and CD45RA(+) T(EM) (T(EMRA)) subsets as early as 4 days after immunization and persisted, particularly in the T(EMRA) subset, up to 2 years after immunization. The majority of HLA-E-restricted CD8(+) cells 28 to 56 days after immunization coexpressed CD107, IFN-gamma, and
TNF-alpha, showing characteristic features of multifunctional T cells. In summary, the multifunctionality and longevity of the HLA-E-restricted CD8 responses observed in this study highlight their significance in adaptive immunity to S. Typhi. Finally, this is the first demonstration, in either animals or humans, of the presence of long-term multifunctional HLA-E-restricted CD8(+) cells after immunization.