Evidence for natural killer cell memory.

Natural killer (NK) cells are generally considered to be part of the innate immune system. Over the past few years, however, evidence has accumulated suggesting that NK cells have certain features that are characteristic of the adaptive immune system. NK cells reportedly respond in an antigen-specific manner to a variety of small molecules and certain viruses, and mediate enhanced responses to these antigens upon secondary exposure. In infections with mouse cytomegalovirus (MCMV), MCMV-specific NK cells undergo clonal expansion, and display increased effector function after the resolution of the infection. In addition, inflammatory conditions resulting from exposure to certain cytokines seem to promote prolonged effector function in NK cells in an antigen-non-specific fashion. Taken together, these studies reveal new aspects of NK biology, and suggest that NK cells, like T and B cells, may carry out memory responses and may also exhibit greater capacity to distinguish antigens than was previously recognized