Eosinophil-mediated mammalian tumor cell cytotoxicity: role of the peroxidase system.

Guinea pig eosinophil peroxidase (EPO) had a potent cytotoxic effect on a line of mouse ascites lymphoma cells designated LSTRA when combined with hydrogen peroxide and a halide. Cytotoxic activity, as measured by 51Cr release, was dependent on each component of the EPO-H2O2-halide system, was completely inhibited by catalase, azide, and aminotriazole, and was partially inhibited by cyanide and gelatin under our experimental conditions. The EPO system was more effective at pH 6.0 than at pH 7.0. At pH 7.0 and with added gelatin (0.005%) to reduce background 51Cr release, iodide was the only effective halide with 10-5 M required. Chloride (0.1 M), although ineffective alone under these conditions, significantly increased cytotoxic activity at otherwise ineffective iodide concentrations (10-6 and 10-7 M). At pH 6.0, iodide, bromide, chloride, or thiocyanate ions could be employed as the halide cofactor. Intact guinea pig eosinophils (96% purity), when stimulated by preopsonized zymosan or phorbol myristate acetate (PMA), were cytotoxic to the LSTRA cells. A chloride-containing medium at pH 7.4 was employed; the further addition of iodide was required for optimum cytotoxic activity with both stimuli and was an absolute requirement when zymosan was used. The cytotoxic effect was inhibited by catalase, azide, and aminotriazole but not by cyanide under our experimental conditions. When PMA-stimulated human neutrophils were employed as the effector cells, the effect on the LSTRA cells was comparable to that of eosinophils except that the addition of iodide to the chloride-containing medium did not significantly increase cytotoxicity; and cyanide, like azide, aminotriazole, and catalase, was completely inhibitory. These findings indicate that eosinophils, like neutrophils, are cytotoxic to mammalian tumor cells when appropriately stimulated and suggest that this cytotoxicity can be mediated by the EPO-H2O2-halide system.