Remembering to remember: T cell memory maintenance and plasticity.

Upon activation, naive T cells give rise to a heterogeneous cell population of effector and memory T cells that mediate antigen clearance and provide long-lived protection from rechallenge. Many of the transcriptional regulators that direct the differentiation of naive T cells to acquire the range of phenotypic and functional characteristics of effector and memory T cells have been described. However, the active programs that maintain the specific subsets of memory T cells are less clear. Here, we discuss recent studies that suggest effector and memory CD8+ T cells exist in cellular 'states' with inherent plasticity. Further, we consider the newly identified role of active transcriptional and epigenetic programming in maintaining the identity of the distinct subsets within the memory population.