The social life of NK cells

Natural killer (NK) cells represent a distinct population of lymphocytes originally identified by its ability to kill transformed cell lines in vitro. It is now clear that these cells also play an important role in the innate immune response against a variety of pathogens, such as virus, bacteria and parasites. In the past few years, different protocols have been developed to activate NK cells ex vivo, allowing a detailed molecular analysis of the interaction of these cells with their cellular targets. NK activity is regulated by signals generated by both inhibitory and stimulatory receptors expressed by target cells. Indeed, recent results indicate that, while major histocompatibility complex class I molecules expressed on target cells inhibit NK lytic activity by engaging surface inhibitory receptors, costimulatory molecules, such as B7-1, B7-2 and CD40, are able to actively trigger NK activity. This review discusses the most recent findings on the role of costimulation on NK activation and forsees the possible consequences of the interaction between NK cells and dendritic cells on the development of an adaptive immune response.