Natural Killer Cell Responses Redefine Immunological Memory

Immunological memory has traditionally been attributed as a unique trait of the adaptive immune system. Nevertheless, there is evidence of immunological memory in lower organisms and invertebrates, which lack an adaptive immune system. Despite their innate ability to rapidly produce effector cytokines and kill virally infected or transformed cells, NK cells also exhibit adaptive characteristics such as clonal expansion, longevity, self-renewal, and robust recall responses to antigenic or non-antigenic stimuli. This review highlights the intracellular and extracellular requirements for memory NK cell generation, and describes the emerging evidence for “memory precursor” NK cells, and their derivation.