Cellular killing of microfilariae of <i>Onchocerca volvulus</i>: eosinophil and neutrophil-mediated immune serum-dependent destruction.

Abstract

Abstract A system that demonstrates in vitro cell-mediated killing of microfilariae of O. volvulus has been developed, using cryopreserved nodule material from endemic areas as the source of microfilariae (Mf). In the presence of immune serum, normal neutrophils and eosinophils, purified using a discontinuous metrizamide gradient, showed enhanced adhesion to Mf, which led to their immobilization and destruction over a 6- to 18-hr period. Peripheral blood mononuclear cells did not attach to or kill Mf. Immunofluorescent staining revealed IgG binding to the surface of microfilariae after incubation in immune serum, but no IgM, IgA, or IgE. The addition of fresh normal human serum (FNHS) increased eosinophil-mediated killing from 50 ± 3 (± SEM) to 68 ± 2% (p &amp;lt; 0.001) and neutrophil killing from 44 ± 3 to 62 ± 3% (p &amp;lt; 0.005). Fresh NHS preferentially enhanced eosinophil-mediated killing, particularly at low concentrations of cells (12 ± 4% killing without FNHS increased to 39 ± 4% with FNHS for eosinophils, while for neutrophils the killing without FNHS was 21 ± 4%, which increased to 26 ± 8%). Microfilariae incubated in immune serum demonstrated complement consumption by a hemolytic C3 assay. Further experiments were done with immune serum plus whole, C4-deficient, and heat-inactivated guinea pig serum. Using purified eosinophils, the absolute increase in the percentages killed were: GPS 40 ± 12, C4D-GPS 26 ± 4; with neutrophils GPS 23 ± 4, C4D-GPS 17 ± 2. The biologic relevance of Mf obtained from cryopreserved nodules was confirmed in experiments using fresh skin-derived Mf and cells from infected individuals in West Africa. These experiments gave similar results except that cell-mediated killing was consistently increased by the addition of fresh serum alone, without immune serum.