Instructing Memory in CD8+ Thymocytes.

T cells can sometimes acquire properties of a memory cell without encountering foreign antigen. Miller et al. now show that development of such 'memory phenotype' (MP) CD8+ T cells starts with recognition of self-ligands in the thymus, and finishes in peripheral tissues. These cells respond rapidly during infection, and infiltrate tumors, where they express high amounts of PD-1.