Maintenance and attrition of T-cell memory.

After antigenic stimulation in the context of "danger signals," naive T cells embark on a programmed, intense expansion phase to counteract the rapid proliferation of pathogens. During the first week of infection, responding T cells undergo > 1000-fold expansion, resulting in the development of large numbers of cells exhibiting potent effector function. As the pathogen (antigen) burden dwindles, a majority of the effectors generated are eliminated by apoptosis, resulting in the survival and maintenance of a small population of antigen-specific cells as long-term memory T cells. Depending on the infection studied, CD8+ T cells appear to differentiate through multiple pathways into resting and effector memory subsets, and require multiple cytokines and cell surface molecules for survival and proliferation. Once generated, the repertoire of memory T cells remains highly vulnerable to attrition during heterologous infections, where homeostatic forces drive deletions in T-cell memory pools to accommodate the entry of new memory T cells. This review will primarily focus on the factors that influence the generation, maintenance, and attrition of memory CD8+ T cells.