Role of myeloperoxidase in the killing of Naegleria fowleri by lymphokine-altered human neutrophils

Previously we have shown that human neutrophils treated with conditioned medium from phytohemagglutinin-stimulated mononuclear leukocytes (sCM) in the presence of antisera have amoebicidal properties for Naegleria fowleri, a pathogenic free-living amoeba. The data now presented show that neutrophils which lack myeloperoxidase (MPO) but have a normal oxygen-dependent respiratory burst could not be altered by sCM to express the amoebicidal activity. Catalase inhibited this amoebicidal activity of sCM-treated neutrophils. Various components and products of the neutrophils were examined for effects on naegleriae. A granule extract was found to have no effect at concentrations up to 100-fold that which killed Salmonella minnesota R595. Hydrogen peroxide appeared to have little effect even at 100 μM. However, in the presence of MPO, H₂O₂ was amoebicidal at 2.5 μM. The generation of amoebicidal activity required the presence of chloride ions. Azide inhibited the effects of the MPO-H₂O₂-Cl^- system. Arginine, a scavenger of hypochlorite, significantly depressed the ability of sCM-treated neutrophils to kill amoebae and also prevented the amoebicidal properties of the MPO-H₂O₂-halide system. These results suggest that the MPO-H₂O₂-halide system is important in the killing of naegleriae by sCM-treated neutrophils and that hypochlorite may be the amoebicidal agent.