Sensitivity of human melanoma cells to adherent leukocytes depends on the ratio between them, the activation status of adherent leukocytes and the metastatic potential of tumor cells

This study examined the interaction of the poorly metastatic human melanoma cell line M4Be and the highly metastatic clone 4 derived from M4Be, with r espect to fresh adherent leukocytes (AL) isolated from 17 different healthy blood donors. These AL contained 80% (73%-93%) monocytes, 15% (6%-20%) B l ymphocytes and 5% (1%-8%) T lymphocytes. The survival of these tumor cells against the stress exerted by these AL was estimated with a clonogenic assa y where isolated tumor cells were co-cultured for 14 days in contact with A L and lipopolysaccharide (LPS). For a given blood donor, AL either stimulat es or inhibits the colony formation of the tumor cells (T) depending on the AL/T ratio, the AL activation status and the metastatic potential of tumor cells. At low AL/T ratios (< 10/1) in the presence of low (8 ng/ml) and tr ace (8 pg/ml) levels of LPS, hydrogen peroxide (H2O2) release is significan tly reduced, and tumor cells significantly increase their colony formation; the feeder effect of AL is suggested to be due to low concentrations of so luble tumor necrosis factor-alpha (TNF-<alpha>). At high AL/T ratios (> 10/ 1), whatever the characteristics of the blood donor, clone 4 is significant ly more sensitive than M4Be to AL activated with medium containing low (8 n g/ml) or high (1,000 ng/ml) levels of LPS; this killing effect is suggested to be due to TNF-alpha, both soluble and membrane-bound, but not to be due to release of H2O2. These data suggest that the regulatory role of AL, whi ch remove the majority of human melanoma cells and stimulate the colony for mation of a small fraction of them, is partly due to TNF-alpha.