Natural killer (NK) cells were initially identified as lymphocytes capable oflys-ing tumor cells without prior sensitization. Meanwhile, numerous studies have established that NK cells play an important role in the immunosurveillance of tumors.1'2 With regard to their antitumor activity, NK cells abide by the law of "missing-self," implying the preferential elimination of cells with a deficient expression of major histocompatibility complex (MHC) class I molecules. This recognition mode constitutes a fail-safe mechanism for MHC class I-restricted tumor recognition by cytotoxic T cells. In addition, triggering of NK-cell cytotoxicity requires stimulation via activating NK receptors that engage their cognate tumor-associated ligands. Apart from the intricate balance of activating and inhibitory signals provided by receptors recognizing ligands on tumor cells,
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