Learning to remember: generation and maintenance of T-cell memory.

Immunologic memory results from a carefully coordinated interplay between cells of the immune system. In this review, we explore various aspects of the nature, generation, and maintenance of T lymphocyte-mediated immunologic memory. In light of the demonstrated heterogeneity of the memory T-cell pool, we hypothesize that subsets of memory T cells instructed to mature to distinct differentiation stages may differ, not only in functional and homing properties, but also in the conditions they require for survival, including antigen persistence and cytokine environment. Hence, according to this hypothesis, distinct memory T-cell subsets result from the nature and timing of the signals provided by the immune environment and occupy distinct niches. Intracellular and extracellular molecular mechanisms that underlie and modulate T-cell memory are discussed.