In addition to their cytotoxic activities, natural killer (NK) cells can have immunoregulatory functions. We describe a distinct “helper” differentiation pathway of human CD56 + CD3 − NK cells into CD56 + /CD83 + /CCR7 + /CD25 + cells that display high migratory responsiveness to lymph node (LN)–associated chemokines, high ability to produce interferon- γ upon exposure to dendritic cell (DC)- or T helper (Th) cell–related signals, and pronounced abilities to promote interleukin (IL)-12p70 production in DCs and the development of Th1 responses. This helper pathway of NK cell differentiation, which is not associated with any enhancement of cytolytic activity, is induced by IL-18, but not other NK cell–activating factors. It is blocked by prostaglandin (PG)E 2, a factor that induces a similar CD83 + /CCR7 + /CD25 + LN-homing phenotype in maturing DCs. The current data demonstrate independent regulation of the “helper” versus “effector” pathways of NK cell differentiation and novel mechanisms of immunoregulation by IL-18 and PGE 2.