Table I.

Main characteristics of the distinct memory CD8+ T cell subsets

Memory SubsetMain CharacteristicsPhenotypic Markers
TcmQuiescence, stemness, quick reactivation and proliferation upon secondary Ag, recirculate between lymphoid organs and bloodCD44+, CD62L+, CCR7+, TCF1hi, IL-7Ra+, CD45RA- (humans)
TscmDevelopment occurs early after clonal activation, phenotypic characteristics resembling naive CD8+ T cells, highly, quiescent, stemness, recirculate between lymphoid organs and bloodCD44lo, CD62L+, CD45RA+ (humans)
TemHeightened capacity to produce cytokines upon secondary Ag, recirculate between lymphoid organs, blood and nonlymphoid organs, shorter lived, decreased stemnessCD44+, CD62L, IL-7Ra+, KLRG1
LLECsMay arise from TEs, highest ability to respond to secondary Ag, do not enter nonlymphoid tissues (mostly confined to blood and spleen red pulp)CD44+, CD62L, IL-7Ra, KLRG1+, CX3CR1hi
TrmDo not recirculate (mostly found in nonlymphoid organs, some in lymphoid organs), quick mediators of secondary immunity to local insults (e.g., barrier infections), high expression of tissue residency markers and transcriptional signatures (e.g., Hobit/Blimp1), can be defined (in mice) by lack of recirculation upon parabiosis surgery or negative intravascular labelingCD44+, CD62L, intravascular Ab (mice), CD69/CD103+ (tissue dependent), KLF2lo, Hobit/Blimp1+ (tissue dependent)