The role of the monocyte cytotoxic factor (CF) in cytolysis of untreated and actinomycin D (Act D)-treated WEHI 164 cells by freshly isolated human adherent mononuclear cells has been investigated in this study. Murine WEHI 164 cells were used as target cells because of their sensitivity to lysis mediated by monocytes and their resistance to natural killer cells. Monocytes as well as monocyte supernatants mediated cytolysis of WEHI 164 cells. Cytolysis was enhanced by Act D treatment of target cells. The addition of lipopolysaccharide to monocytes accelerated the progression of cytolysis of Act D-treated WEHI 164 cells mediated by monocytes. A polyclonal rabbit antiserum against CF inhibited the cytolytic activity of monocytes and monocyte supernatants against untreated as well as Act D-treated WEHI 164 cells. At low effector:target ratios, the cytolysis was totally abrogated by CF antiserum. Depletion of natural killer cells from adherent cells by the monoclonal antibody Leu 11b and rabbit complement did not reduce cytolysis of Act D-treated WEHI 164 cells. Immunofluorescence microscopy revealed that CF antiserum stained the plasma membrane of freshly isolated monocytes, suggesting that CF is a membrane-associated molecule. Our data indicate that CF is an important effector molecule in cytolysis mediated by freshly isolated monocytes against untreated and Act D-treated WEHI 164 cells.

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This work was supported by grants from the Norwegian Society for Fighting Cancer (Norsk Forening til Kreftens Bekjempelse) and by the Norwegian Cancer Society (Landsforeningen mot Kreft).

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