Susceptibility of NB‐I Neuroblastoma Cells to Tumoricidal Activity of Monocytes Activated by γ‐Interferon

The purpose of this study was to examine the susceptibility of NB‐I human neuroblastoma cells to direct cellular cytotoxicity mediated by peripheral blood monocytes from pediatric cancer patients receiving chemotherapy. Nonactivated monocytes from patients showed spontaneous cytotoxicity to NB‐I neuroblastoma cells (37 ± 18%) but only marginal cytotoxicity to A375 melanoma cells (21 ± 14%) at the effector:target cell ratio of 20:1. This spontaneous cytotoxicity to NB‐I cells was observed only after >24 h of cocultivation and was proportional to the effector:target cell ratio. Activation of monocytes by recombinant human interferon γ (rIFN) (1×10(4) U/ml) consistently and strongly enhanced their tumoricidal activity to NB‐I cells (87 ± 6%) and this tumoricidal activity was even superior to that observed against A375 cells, which are known to be extremely sensitive to lysis by activated monocytes. In contrast, activation of monocytes by lipopolysaccharide (LPS, 1 μg/ml) had no effect on monocyte‐mediated lysis of NB‐I cells, while A375 cells were equally lysed by rIFN‐ and LPS‐activated monocytes, thus suggesting that different mechanisms are involved in the monocyte‐mediated lysis of A375 melanoma and NB‐I neuroblastoma cells. Susceptibility of the neuroblastoma cell line to monocyte‐mediated cytotoxicity has not been reported so far and our results may have some clinical implication if this observation can be extended to other neuroblastoma cell lines as well.