NK cells at the interface between innate and adaptive immunity.

In recent years a novel concept has emerged indicating that the actual role of natural killer (NK) cells is not confined to the destruction of virus-infected cells or tumors. Indeed, different NK subsets exist that display major functional differences in their cytolytic activity, cytokine production and homing capabilities. In particular, CD56(high) CD16(-) NK cells that largely predominate in lymph nodes, have little cytolytic activity but release high levels of cytokines whereas CD56(low) CD16(+) NK cells that predominate in peripheral blood and inflamed tissues, display lower cytokine production, but potent cytotoxicity. The latter is characterized by granule polarization and exocytosis of various proteins including perforin and granzymes that mediate target cell killing. The recruitment of CD56(low) CD16(+) NK cells into inflamed peripheral tissues is orchestrated by various chemochines including the newly identified Chemerin. At these sites, NK cells, upon engagement of different triggering receptors become activated and upregulate their cytokine production and cytotoxicity after interaction with myeloid dendritic cells (DCs). Importantly, during this interaction NK cells also mediate the 'editing' of DCs undergoing maturation. This process appears to play a crucial role in shaping both innate and adaptive immune responses. Indeed, only DCs undergoing this NK-mediated quality control would become fully mature and capable of inducing priming of protective Th1 responses.