Stem, Effector, and Hybrid States of Memory CD8+ T Cells

CD8⁺ T cell immunological memory of past antigen exposure can confer long-lived protection against infections or tumors. The fact that CD8⁺ memory T cells can have features of both naïve and effector cells has forced the field to struggle with several conceptual questions about the developmental origin of the cell and, consequently, the mechanism(s) that contribute to memory development. Here, we discuss recent conceptual advances in our understanding of memory T cell development that incorporate data describing a hybrid stem and/or effector state of differentiation. We theorize that the mechanisms involved in developing these cells could be mediated, in part, through epigenetic programs. Finally, we consider the potential therapeutic implications of inducing and/or utilizing such hybrid cells clinically.