FIG. 1.

CD8⁺ T cell memory subsets and differentiation models. (A) At least four different memory CD8⁺ T cell subsets have been proposed: stem cell memory (T_SCM), central memory (T_CM), effector memory (T_EM), and tissue-resident memory (T_RM) cells. Memory subsets display distinct circulations and tissue compartmentalization patterns. (B) Three proposed models of memory differentiation: (i) the linear model proposes the progressive loss of memory potential as the CD8⁺ T cells acquire effector functions according to the strength/duration of TCR signaling or the extent of antigenic stimulation. (ii) The circular model proposes memory CD8⁺ T cells undergo an obligatory effector stage before de-differentiating in memory CD8⁺ T cells. (iii) The asymmetric division model proposes an unequal distribution of regulatory molecules, with one daughter cell displaying a greater memory potential, while the other daughter cells have a greater effector potential. TCR, T cell receptor.