Memory CD8 T cells are divided into two subsets: central memory (T_CM) and effector memory (T_EM). This study shows that T_CM have a greater capacity to persist in vivo and are more efficient in mediating protective immunity due to their increased proliferative potential. Following antigen clearance, T_EM convert to T_CM, and this conversion is programmed within the first week after immunization. The study proposes that T_CM and T_EM are part of a continuum in a linear differentiation pathway from naive to effector to T_EM to T_CM. T_CM are more effective in protective immunity than T_EM, as they can undergo efficient homeostatic proliferation. The study used two models of T cell immunity, acute infection with lymphocytic choriomeningitis virus (LCMV) or the intracellular bacterium Listeria monocytogenes (LM), to examine the protective capacity of T_CM and T_EM. The results show that T_CM provide more effective protection than T_EM, and that the conversion of T_EM to T_CM is programmed during the initial phase of antigen exposure. The study also demonstrates that T_CM have a greater capacity to proliferate and persist in vivo, and that they acquire the ability to undergo efficient homeostatic turnover. The findings suggest that T_CM are the "true" memory cells, as they exhibit both hallmark characteristics of memory T cells: long-term persistence in vivo by self-renewal and the ability to rapidly expand upon reencounter with pathogen. The study also shows that the rate of T_EM to T_CM conversion can vary depending on the nature of the immunization and that this conversion rate is programmed during the initial period of encounter with antigen in vivo. The study concludes that the differentiation of memory T cells follows a linear pathway from naive to effector to T_EM to T_CM, and that T_CM are the final product of this differentiation process. The study also highlights the importance of antigen-driven proliferation in the development of memory T cells and the role of homeostatic proliferation in maintaining memory T cell populations. The findings suggest that memory T cell differentiation is a gradual process and that memory cells only develop several weeks after clearance of the acute infection. The study also shows that the conversion of T_EM to T_CM is primarily an antigen-driven process, and that the duration of this conversion is not constant but is imprinted during effector generation and varies depending on the magnitude of the initial stimulation. The study provides important insights into the lineage relationships and protective immunity of memory CD8 T cell subsets, and highlights the importance of antigen-driven proliferation in the development of memory T cells.Memory CD8 T cells are divided into two subsets: central memory (T_CM) and effector memory (T_EM). This study shows that T_CM have a greater capacity to persist in vivo and are more efficient in mediating protective immunity due to their increased proliferative potential. Following antigen clearance, T_EM convert to T_CM, and this conversion is programmed within the first week after immunization. The study proposes that T_CM and T_EM are part of a continuum in a linear differentiation pathway from naive to effector to T_EM to T_CM. T_CM are more effective in protective immunity than T_EM, as they can undergo efficient homeostatic proliferation. The study used two models of T cell immunity, acute infection with lymphocytic choriomeningitis virus (LCMV) or the intracellular bacterium Listeria monocytogenes (LM), to examine the protective capacity of T_CM and T_EM. The results show that T_CM provide more effective protection than T_EM, and that the conversion of T_EM to T_CM is programmed during the initial phase of antigen exposure. The study also demonstrates that T_CM have a greater capacity to proliferate and persist in vivo, and that they acquire the ability to undergo efficient homeostatic turnover. The findings suggest that T_CM are the "true" memory cells, as they exhibit both hallmark characteristics of memory T cells: long-term persistence in vivo by self-renewal and the ability to rapidly expand upon reencounter with pathogen. The study also shows that the rate of T_EM to T_CM conversion can vary depending on the nature of the immunization and that this conversion rate is programmed during the initial period of encounter with antigen in vivo. The study concludes that the differentiation of memory T cells follows a linear pathway from naive to effector to T_EM to T_CM, and that T_CM are the final product of this differentiation process. The study also highlights the importance of antigen-driven proliferation in the development of memory T cells and the role of homeostatic proliferation in maintaining memory T cell populations. The findings suggest that memory T cell differentiation is a gradual process and that memory cells only develop several weeks after clearance of the acute infection. The study also shows that the conversion of T_EM to T_CM is primarily an antigen-driven process, and that the duration of this conversion is not constant but is imprinted during effector generation and varies depending on the magnitude of the initial stimulation. The study provides important insights into the lineage relationships and protective immunity of memory CD8 T cell subsets, and highlights the importance of antigen-driven proliferation in the development of memory T cells.