Eosinophil peroxidase (EPO), a cationic
protein purified from horse blood, adhered to four different types of
tumor cells, markedly potentiating their lysis by preformed or enzymatically generated H(2)0(2) (up to 76-fold, as assayed in serum-containing tissue culture medium without supplemental halide). Similarly, compared with uncoated
tumor cells, EPO-coated
tumor cells were up to 32 times more sensitive to lysis when incubated with macrophages or granulocytes whose respiratory burst was triggered by PMA. However, EPO-coated
tumor cells were also readily lysed by bacillus Calmette- Guerin-activated macrophages in the absence of exogenous triggering agents. This spontaneous cytolysis was rapid (50 percent at 2 h) and potent (50 percent lysis at macrophage/
tumor cell ratios of 1.5 to 4.6), and was observed with both a
peroxide-sensitive
tumor (TLX9) and a
peroxide-resistant
tumor (NK
lymphoma). Under the conditions used, neither EPO alone nor macrophages alone were spontaneously cytolytic. Neither EPO nor EPO-coated
tumor cells triggered a detectable increment in H(2)0(2) release from macrophages. Nonetheless, spontaneous macrophage-mediated cytolysis of EPO- coated
tumor cells was completely inhibitable by
catalase (50 percent inhibition, 23 U/ml), although not by heated
catalase, indicating a requirement for H(2)0(2). Cytolysis was also completely inhibitable by
azide (50 percent inhibition, 2.6 X 10 -5 M), indicating a requirement for enzymatic activity of EPO. Thus, a cytophilic
peroxidase from eosinophils and H(2)0(2) spontaneously released from activated macrophages interacted synergistically in a physiologic medium to destroy
tumor cells.