Fig 1.
Key features of a primary T-cell response after vaccination. In the human T-cell repertoire, approximately 1 in every 10,000 naive cells will be a vaccine (VAX)-specific cell. Upon exposure to vaccination, these cells become activated through engagement of their TCR, and other costimulatory receptors, and undergo massive clonal expansion. During this expansion phase, activated naive cells differentiate into short-lived effector cells or memory precursor cells (MPECs). TFH cells are generated, likely in a similar manner as MPECs, and provide critical help to B cells for the generation of high-affinity vaccine-specific antibodies. After resolution of vaccine antigen, most effector cells die. A small subset of MPECs survives to become long-lived memory T cells. These memory cells provide protection against subsequent infection or booster vaccination, possibly for decades, via a higher frequency of vaccine-specific cells, their poised effector state, and potential tissue localization. Recall responses after booster vaccination follow the same scheme, however, starting at higher initial frequency of VAX-specific T cells.