Vaccination of human subjects expands both specific and bystander memory T cells but antibody production remains vaccine specific.

Human subjects maintain long-term immunologic memory against infective organisms but the mechanism is unclear. CD4+ T-helper memory (Thmem) cells are pivotal in controlling humoral and cellular responses, therefore their longevity and response to vaccination are critical for maintenance of protective immunity. To probe the dynamics of the Thmem-cell response to antigenic challenge, we investigated subjects following a booster injection with tetanus toxoid (TT). Expansion of TT-specific Thmem cells and cytokine production showed complex kinetics. Strikingly, parallel expansion and cytokine production occurred in pre-existing Thmem cells specific for 2 other common antigens: purified protein derivative of tuberculin and Candida albicans. Bystander expansion occurred in Thmem but not in Thnaive cells. Antibody production against TT peaked approximately 2 weeks after vaccination and gradually declined. However, pre-existing antibody against the other antigens did not change. It appears that although all Thmem cells are readily stimulated to expand, antibody responses are controlled by antigen availability. These findings relate to the maintenance of memory and have consequences for assessments of specific T-cell responses to vaccination.