The majority of highly activated CD4+ T cell effectors die after antigen clearance, but a small number revert to a resting state, becoming memory cells with unique functional attributes. It is currently unclear when after antigen clearance effectors return to rest and acquire important memory properties. We follow well-defined cohorts of CD4+ T cells through the effector-to-memory transition by analyzing phenotype, important functional properties, and gene expression profiles. We find that the transition from effector to memory is rapid in that effectors rested for only 3 days closely resemble canonical memory cells rested for 60 days or longer in the absence of antigen. This is true for both Th1 and Th2 lineages, and occurs whether CD4+ T cell effectors rest in vivo or in vitro, suggesting a default pathway. We find that the effector memory transition at the level of gene expression occurs in two stages: a rapid loss of expression of a myriad of effector-associated genes, and a more gradual gain of expression of a cohort of genes uniquely associated with memory cells rested for extended periods. To understand the molecular basis for the rapid and robust memory T cell responses, we examined gene expression and chromatin modification by histone H3 lysine 9 (H3K9) acetylation in resting and activated human nave and memory CD8+ T cells. We found that, although overall gene expression patterns were similar, a number of genes are differentially expressed in either memory or nave cells in their resting and activated states. To further elucidate the basis for differential gene expression, we assessed the role of histone H3K9 acetylation in differential gene expression. Strikingly, higher H3K9 acetylation levels were detected in resting memory cells, prior to their activation, for those genes that were differentially expressed following activation, indicating that hyperacetylation of histone H3K9 may play a role the selective and rapid gene expression of memory CD8+ T cells. Consistent with this model, we showed that inducing high levels of H3K9 acetylation resulted in an increased expression in nave cells of those genes that are normally expressed differentially in memory cells. Together, these findings suggest that differential gene expression mediated at least in part by histone H3K9 hyperacetylation may be responsible for the rapid and robust memory CD8+ T cell response.
