IntroductionNatural killer (NK) cells and dendritic cells (DCs) are 2 specialized cell types of the innate immunity that act as liaisons between innate and Ag-specific adaptive immune responses. [1][2][3][4] DCs function as sentinels of the immune system and display the unique capability of inducing primary immune responses by providing all the signals necessary to prime naive T cells. 5 NK cells play an active role in innate responses against viruses, bacteria, and tumors thanks to their potent cytotoxic activity and rapid production of cytokines, particularly interferon (IFN)-␥ and tumor necrosis factor (TNF)-␣. 6,7 Because of its important role in shaping the immune response, the crosstalk between NK cells and DCs has been a field of extensive study in recent years. [8][9][10][11][12][13][14][15] It has been shown that the cytolytic activity of fresh NK cells is strongly up-regulated by the interaction with monocyte-derived DCs (MoDCs) that had been exposed to maturation stimuli such as lipopolysaccharide (LPS), interferon (IFN)-␣, or Mycobacterium tuberculosis. [9][10][11][12] Reciprocally, upon interaction with immature DCs (iDCs) in the presence of maturation stimuli, fresh NK cells can strongly enhance DC maturation and cytokine production. 8,10,11,14 In addition, IL-2-activated NK cells could directly induce DC maturation and enhance their ability to stimulate alloproliferative responses of naive CD4 ϩ T cells. 10 In this case, DC maturation occurs at low NK/DC ratios and is dependent on cell-to-cell interactions mediated via the NKp30 and DNAM-1 NK receptors and the release of TNF-␣ and IFN-␥ by NK cells. 8,[13][14][15][16] Regarding the effect exerted by DCs on NK cells, most NK cells become partially activated upon interaction with pathogen-activated DCs, as revealed by the expression of CD69 and by the increase of their cytotoxic activity against different targets.In general, major histocompatibility complex (MHC) class I-deficient aberrant cells are susceptible to NK cell-mediated lysis, whereas normal cells are protected from NK cells because they express MHC class I molecules. iDCs represent a remarkable exception. In an autologous setting, they are highly susceptible to lysis by activated polyclonal NK-cell populations but become resistant when they undergo maturation. 17 Killing of iDCs is confined to those NK cells that lack killer immunoglobulinlike receptor (KIR) specific for self-human leukocyte antigen (HLA) class I alleles and express the HLA-E-specific CD94/ NKG2A inhibitory receptor. 18 Based on these data, an additional function of NK cells would be to keep in check the quality of DCs undergoing maturation in peripheral tissues and to control the amplitude of DC responses and subsequent T-cell priming. This function is based mostly on the ability of NK cells to selectively kill DCs that do not express sufficient amounts of HLA-E at the cell surface. 18,19 Because iDCs express very low amounts of HLA-E molecules that are acquired progressively during DC maturation, NK cells would operate...