Effector mechanisms of Human Monocyte-mediated Tumor Cytotoxicity in Vitro: Parameters of Induction of Cytotoxins from Peripheral Blood Monocytes Isolated by Counterflow Elutriation

Human peripheral blood monocytes, isolated in high purity by centrifugal counterflow elutriation from normal donors, were stimulated in vitro to release cell toxins, herein termed human monocyte toxin(s) (HMT). Bacterial lipopolysaccharide, the lipophilic 6-O-stearoyl derivative of muramyl dipeptide, and 4β-phor-boMβ-myristate-13α-acetate served as effective induction signals. Induction involved a sequence of transcription, translation, and secretion, all necessary for HMT synthesis and release into the supernatant as determined by blocking of these functions with the drugs actinomycin D, cycloheximide and monensin, respectively; HMT levels reached a peak within 4–6 h and thereafter declined. The levels of HMT produced varied considerably from donor to donor; one parameter causing this variability appeared to be the plateletapheresis history of the donor. Monocytes from donors subjected to pheresis for the first time were responsive to induction signals immediately after adherence and could not be brought to a higher state of priming for HMT production by further in vitro culture for up to 9 days, with or without recombinant human 7-interferon. In contrast, monocytes from donors who had recently undergone pheresis (up to 1 wk earlier) were poorly responsive initially to triggering with lipopolysaccharide; however, these cells could be brought to a highly primed state for HMT production by a combination of culture in vitro for several days and a subsequent 24-h exposure to recombinant 7-interferon (0.1-1.0 units/ml). These primed cells could then be effectively triggered by lipopolysaccharide to release HMT. HMT was found to be cytotoxic (cytostatic/cyto-lytic) for human and murine tumor cells in vitro.