Eosinophils versus Neutrophils in Host Defense: KILLING OF NEWBORN LARVAE OF TRICHINELLA SPIRALIS BY HUMAN GRANULOCYTES IN VITRO

Eosinophil leukocytes have been reported to have a major role in host defense against invasive, migratory phases of helminth infestations, yet the relative larvicidal abilities of eosinophils and neutrophils have not been thoroughly examined. This study examined the killing of newborn (migratory phase) larvae of Trichinella spiralis during incubation by human granulocytes in vitro. The assay employed cultue of larvae with cells, sera, and reagents in microtiter wells with direct counting of surviving larvae after incubation. Killed larvae appeared to be lysed. Verification of the microplate assay was obtained by demonstrating complete loss of infectivity of larvae incubated with leukocytes and immune serum. In the presence of optimal immune serum concentrations, purified neutrophils or eosinophils achieved >/=95% killing of larvae at cell:larva ratios of 2,000:1 or greater. Fresh normal serum prompted slight (19%) killing by leukocytes at a cell:larva ratio of 9,000:1. Cells plus heat-inactivated normal serum and all sera preparations in the absence of leukocytes killed <8% of the larvae. The activity of immune serum was opsonic. Cells adhered to larvae that had been preincubated in immune serum, and immunofluorescent studies indicated that such preopsonized larvae were coated with immunoglobulin (Ig)G. However, preopsonized larvae lost opsonic activity and surface IgG during incubation for 3 h in medium lacking immune serum.The rate of killing was dependent on the cell:larva ratio; at high leukocyte concentrations (4,200:1), 99% were killed within 7 h; at lower cell:larva ratios, killing increased steadily during a 20-h incubation period. Killing was inhibited by 20 mug catalase, 5 mug/ml cytochalasin B, or 5muM colchicine, but was unchanged by superoxide dismutase and was enhanced by azide or cyanide. Leukocytes from a patient with chronic granulomatous disease, lacking ability to mount a normal oxidative response, demonstrated a markedly suppressed larvicidal effect.The data indicate that neutrophils are at least as effective as eosinophils in the killing of newborn larvae of T. spiralis. The killing appeared to be mediated by the oxidative metabolic burst with its generation of hydrogen peroxide.