Memory-like responses of natural killer cells.

Natural killer (NK) cells are lymphocytes with the capacity to produce cytokines and kill target cells upon activation. NK cells have long been categorized as members of the innate immune system and as such have been thought to follow the 'rules' of innate immunity, including the principle that they have no immunologic memory, a property thought to be strictly limited to adaptive immunity. However, recent studies have suggested that NK cells have the capacity to alter their behavior based on prior activation. This property is analogous to adaptive immune memory; however, some NK cell memory-like functions are not strictly antigen dependent and can be demonstrated following cytokine stimulation. Here, we discuss the recent evidence that NK cells can exhibit properties of immunologic memory, focusing on the ability of cytokines to non-specifically induce memory-like NK cells with enhanced responses to restimulation.