Activation of natural killer cells with interleukin 2 (IL-2) and IL-12 increases perforin binding and subsequent lysis of tumour cells

Natural killer (NK) cells can lyse a variety of different tumour cells by e xocytosis of perforin, subsequent binding of perforin to the target cell me mbrane and formation of lytic pores. Some tumour cells, however, are resist ant to cellular cytotoxicity. Using the NK-resistant tumour cell lines ML-2 , MONOMAC-1, RPMI and L540Cy, we demonstrated that activation of NK cells w ith interleukin 2 (IL-2) and IL-12 resulted in significant lysis of these t umour targets. To investigate the underlying mechanisms, we isolated the cy totoxic granules from non-activated and IL-2-/IL-12-activated NK cells and compared the killing of K562 leukaemia cells (sensitive to NK cell-mediated lysis) and ML-2 leukaemia cells (resistant to NY, cell-mediated lysis). In contrast to K562 cells, which were easily killed by NK-cell granules, ML-2 cells were resistant to granules from non-activated NK cells. However, gra nules from INK cells activated with IL-2 and IL-12 were able to induce sign ificant tumour cell lysis. Cell death of both K562 and ML-2 cells by granul es from activated NIC cells was completely blocked by anti-perforin antibod ies, indicating that perforin mainly accounts for the lysis induced by NK g ranules. Comparing granules from non-activated and IL-2-/IL-12-activated NI C cells, the increased cell death of ML-2 cells was caused by an improved b inding of perforin to the target cell membrane. Functional assays, however, indicated that the differences in perforin binding were not as a result of an augmented production of perforin by activated NK cells. We conclude tha t activation of NK cells results in an increased binding of perforin and su bsequent lysis of tumour cells.