Lysis of Tumor Cells by Human Blood Monocytes by a Mechanism Independent of Activation of the Oxidative Burst

Tumorilytic human blood monocytes recognize and destroy neoplastic cells by a mechanism that is nonphagocytic and requires cell-to-cell contact. The mechanism of cytolysis subsequent to binding is controversial. Release of reactive oxygen intermediates by activated rodent macrophages has been suggested as an important mechanism for tumor cell lysis in some short-term cytotoxicity assays. We examined whether oxygen intermediates are also responsible for mediating the lysis of adherent human tumor cells in a long-term (72-h) tumoricidal assay. Human blood monocytes were incubated with medium, concanavalin A-stimulated lymphokine [macrophage-activating factor (MAF)], lipopolysaccharide endotoxin, or human recombinant γinterferon for 24 h prior to the addition of [¹²⁵l]iododeox-yuridine-labeled A375 melanoma cells. The following evidence indicated that monocyte-mediated tumor cell lysis was independent of superoxide anion (O2-) and H₂O₂ production: (a) although human blood monocytes incubated for 24 h with y interferon produced twice as much O_2- as control or MAF-treated monocytes, y interferon did not activate monocyte tumoricidal activity unless combined with lipopolysaccharide endotoxin, 0.2 ng/ml or more; (b) incubating the monocytes with 10 nM phorbol myristate acetate for 0.5 h stimulated O_2- production but no cytotoxicity; (c) the cytolytic activity of MAF-treated monocytes was not decreased in the presence of catalase or superoxide dismutase; and (d) finally, peripheral blood monocytes were isolated from six patients with chronic granulomatous disease, activated by MAF or lipopolysaccharide endotoxin, and then assayed for tumoricidal activity. While these activated chronic granulomatous disease monocytes did not produce O_2- or H₂O₂, tumor cell lysis was normal in all six patients. Hence, lysis of tumor cells in a 72-h assay is not dependent upon the generation of O_2- and/or H₂O₂ and is intact in chronic granulomatous disease monocytes.