Investigating the lysis of small-cell lung cancer cell lines by activated natural killer (NK) cells with a fluorometric assay for NK-cell-mediated cytotoxicity

Activation of natural killer (NK) cells with interleukin-2 (IL-2) and IL-12 leads to an enhanced lysis of tumour cells. We investigated the ability of NK cells, with or without prior activation, to lyse a variety of small-cel l lung cancer (SCLC) target cells. Specific lysis was measured with a fluor ometric assay for NK-cell-mediated cytotoxicity: target cells were labelled with 3,3'-dioctadecyloxacarbocyanine, a green membrane dye. After co-incub ation with NK cells, dead target cells were stained with propidium iodide, a red DNA dye that only penetrates dead cells. Of all eight SCLC cell lines tested, three were susceptible to lysis by non-activated NK cells, three w ere only susceptible to lysis by NK cells activated with IL-2 and IL-12 and two were not even susceptible to lysis by activated NK cells. The differen ces in target cell susceptibility showed no correlation with the expression of MHC-I on the surface of the target cells or with the expression of the adhesion molecules CD50, CD54, CD58 or CD102. Comparing the kinetics of the lysis of one SCLC cell line sensitive to non-activated NK cells and one se nsitive only to activated NK cells, we found that maximum lysis of the form er was obtained after 1 h, whereas significant lysis of the latter was only obtained after 4 h of incubation. This might be due to different mechanism s engaged in target cell lysis.