Figure 2. Model for the generation of human memory T cell subsets.

A schematic model for differentiation of circulating and tissue-resident memory T cell subsets. Progressive differentiation of the three major circulating subsets — stem cell memory T cells (T_SCM cells), central memory T cells (T_CM cells) and effector memory T cells (T_EM cells) — from activated naïve T cells is shown relative to the extent of antigen exposure. Effector T cells (T_Eff cells) represent terminally differentiated cells, and death is one outcome of increased antigen exposure and proliferation. Naïve, T_SCM and T_CM cells circulate and migrate to lymphoid tissue, whereas T_EM and T_Eff cells are the subsets with the capacity to traffic to peripheral tissues. Tissueresident memory T cells (T_RM cells) in peripheral tissue sites may derive from either T_EM or T_Eff cells that migrate to these sites via tissue-specific influences. It is possible that T_CM cells could develop into T_RM cells in lymphoid sites (dotted line). T_RM cells in peripheral compartments are likely terminally differentiated since they do not circulate or convert to other memory T cell subsets.