Lymphokine activated killer (LAK) cells were cocultivated from 2 to 6 days with WM266 metastatic melanoma cells maintained as nodules in organotypic culture. The LAK cells in suspension were allowed to deposit freely on the nodule surface from where they could infiltrate spontaneously into the nodules. Immunohistochemical studies were done to localize the LAK cells as well as electron microscopical observations for effector/target membrane contacts. Proliferation of the nodules was tested and also that of the LAK cells after coculturing using tritiated thymidine incorporation into DNA. Cell death was determined by arrest of thymidine incorporation and total nodule disintegration. Infiltration rate of LAK cells into the nodules was low: after coculturing 5% of the nodule cells were LAK cells. Although close membrane contacts and cytoplasmic fusions between effector and target cells leading to tumor cell apoptosis were observed, this direct cytolytic process seemed to be too infrequent for the induction of total nodule disintegration at day 6. Therefore, the indirect pathway to cytolysis might be predominant implying, among other cytokines, soluble TNF. On the other hand, LAK cell proliferation diminished strongly after coculturing (down to 11%) but the cytotoxicity was significantly enhanced (18% higher) suggesting an enhancement of differentiation. This might account for the peculiar efficacy of LAK cells on melanomas in vivo and it would be of interest to study this phenomenon further.

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