Human Monocyte‐Released Cytotoxic Factor

Human lymphokine‐activated monocytes release a cytotoxic protein factor (CF). The cytostatic activity of CF was potentiated by the DNA and RNA synthesis inhibitors actinomycin D, daunomycin, and mitomycin C. These inhibitors increased the sensitivity for detecting CF approximately 10‐fold, and this is of great practical value when assaying CF. The CF‐potentialing inhibitors had a similar effect on the cell cycle distribution in that they all induced an accumulation of cells in the S and G 2 phases of the cell cycle. Cytolytic activity was shown to be associated with CF, and this activity was also greatly potentiated by daunomycin and actinomycin D. The two other inhibitors studied, cycloheximide and 5‐fIuorouracil (5‐FU), had an adverse effect on the cytostatic activity of CF. These two inhibitors reduced the sensitivity of the assay for CF about fivefold. Cycloheximide had no apparent effect on the relative cell cycle distribution, whereas 5‐FU induced an accumulation of cells in the G 1 phase. Of the inhibitors studied, only those that induced an accumulation of cells in the S and G 2 phases of the cell cycle potentiated the cytotoxic activity of CF. suggesting that CF may preferentially act in these parts of the cell cycle.