Molecular mechanisms for generation and maintenance of memory CD8T cells following bacterial infection.

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During the course of acute infection with an intracellular pathogen, antigen-specific T cells proliferate in the expansion phase, and then most of the T cells die by apoptosis in the following contraction phase, but the few that survive become memory cells and persist for a long period of time. Memory CD8T cells are activated rapidly against re-infection and eliminate pathogens. Using an adoptive transfer system of OT-I cells expressing OVA_<257-264>/K^b-specific T cell receptor into control, IL-15 knockout (KO) and IL-15 transgenic (Tg) mice followed by challenge with recombinant Listeria monocytogenes expressing OVA, we found the roles of IL-15 in generation, maintenance and reactivation of memory CD8T cells as follows :(1) Effector phase : IL-15 is dispensable for generation of effector CD8T cells following bacterial infection.(2) Contraction phase : IL-15 plays a critical role in protecting effector CD8^+T cells from apoptosis during the contraction phase following a microbial infection via inducing anti-apoptotic molecules.(3) Maintenance phase : IL-15 plays an important role in long-term maintenance of memory CD8^+ T cells through induction of homeostatic proliferation.(4) Reactivation : IL-15 plays an important role in early activation of memory CD8^+ T cells through induction of cytotoxic molecules upon re-infection.