The kiss of death: interrupted by NK-cell close encounters of another kind.

An intimate but deadly encounter between natural killer (NK) cells and transformed cells results in the delivery of cytotoxic molecules into tumor cells. New evidence reveals another intimate association, this time between NK cells and regulatory T cells, but this encounter opposes NK-cell tumoricidal activity. Through a contact-dependent mechanism involving transforming growth factor-beta, regulatory T cells provide an escape mechanism from NK-cell-mediated tumor surveillance.