The dynamic co-evolution of memory and regulatory CD4+ T cells in the periphery.

Whereas memory T cells are required to maintain immunity, regulatory T cells have to keep the immune system in check to prevent excessive inflammation and/or autoimmunity. Both cell types must be present during the lifetime of the organism. However, it is not clear whether both subsets are regulated in tandem or independently of each other, especially because thymic involution severely restricts the production of T-cell populations during ageing. In this Opinion article, we discuss recent evidence in both mice and humans that supports the hypothesis that some CD4(+)CD25(+)FOXP3(+)regulatory T cells can differentiate from rapidly proliferating memory T cells in the periphery.