Abstract
The effect of actinomycin D on target susceptibility to human blood natural killer (NK) cells and monocytes was analysed in direct cell‐mediated and their cytotoxic factor‐mediated cytotoxicity assays. Treatment of K562 cells with actinomycin D reduced their susceptibility to lysis by non‐adherent lymphocytes and Percoll‐purified large granular lymphocytes (LGL) in a 4‐hr ^51^Cr‐release assay, without affecting their sensitivity to monocytes purified by adherence to autologous serum‐coated plastic surfaces. The drug treatment caused no shift in the kinetics of cytotoxicity. In the target binding assay LGL formed fewer conjugates with actinomycin‐D‐treated K562 cells than with untreated ones, while the binding of monocytes to targets was not reduced by the drug treatment of K562 cells. The cold target competition assay revealed that actinomycin‐D‐treated cold K562 cells showed less successful inhibition than untreated cold K562 cells. Lymphocytes and monocytes could be induced to release soluble cytotoxic factors, termed natural killer cytotoxic factors (NKCF) and monocyte cytotoxic factors (MCF), respectively, when co‐cultured with K562 cells. Both cytotoxic factors lysed NK‐sensitive target cells in a 48‐hr assay. Actinomycin‐D‐treated K562 cells reduced or abolished the ability to stimulate the release of NKCF from lymphocytes, whereas they induced MCF secretion from monocytes as effectively as untreated ones. On the other hand, actinomycin D treatment of K562 cells enhanced their susceptibility to NKCF and MCF. This actinomycin‐D‐induced augmentation of target sensitivity to the cytotoxic factors was restricted to NK‐sensitive target cells (K562 and Molt‐4). NK‐resistant target cells (Raji, YAC‐I, EL4 and T blasts) were not lysed by NKCF and MCF even after they were treated with actinomycin D. The capacity of K562 cells to bind NKCF and MCF was not altered by actinomycin D. Treatment of the adherent cell population with OKMI or Leu‐MI plus complement abrogated both cell‐mediated cytotoxicity and MCF production, while Leu‐11b plus complement was ineffective. These results suggest that the effect of actinomycin‐D treatment can be used to distinguish the two distinct types of blood mononuclear cells with natural cytotoxicity, NK cells and monocytes, and that each effector type recognizes different plasma membrane moieties of NK target cells, although the cytotoxic factors released from each effector cell similarly bind to and lyse the target cells.