Opportunities and limitations of natural killer cells as adoptive therapy for malignant disease

Although natural killer (NK) cells can be readily generated for adoptive therapy with current techniques, their optimal application to treat malignant diseases requires an appreciation of the dynamic balance between signals that either synergize with or antagonize each other. Individuals display wide differences in NK function that determine their therapeutic efficacy. The ability of NK cells to kill target cells or produce cytokines depends on the balance between signals from activating and inhibitory cell-surface receptors. The selection of NK cells with a predominant activating profile is critical for delivering successful anti-tumor activity. This can be achieved through selection of killer immunoglobulin-like receptor–mismatched NK donors and by use of blocking molecules against inhibitory pathways. Optimum NK cytotoxicity may require licensing or priming with tumor cells. Recent discoveries in the molecular and cellular biology of NK cells inform in the design of new strategies, including adjuvant therapies, to maximize the cytotoxic potential of NK cells for adoptive transfer to treat human malignancies.