Onchocerca volvulus: in vitro cytotoxic effects of human neutrophils and serum on third-stage larvae

The cytotoxic effects of neutrophils co-cultured with infective third-stage larvae (L3) and autologous serum from three groups of individuals: infected (INF), non-patent endemic normals (EN) and non-endemic controls (NEC), were compared using a MTT (3-(4,5 dimethylthiazol-2yl)-2,5 diphenyl tetrazolium bromide) reduction assay as a biochemical parameter of larval viability. The MTT assay permitted the detection of L3 which had suffered varying levels of necrosis. The observations from this study indicate that infected individuals and endemic normals are capable of mounting a specific immune response against the infective L3. This is based on our findings of i) significantly greater numbers of L3 showed necrosis in neutrophil cultures of the INF and EN compared to those from the NEC, ii) a superior ability of cultures from these individuals to inhibit the molting of L3 to fourth-stage larvae, iii) IgG antibodies reactive with the surface of L3, detected by immunofluorescence in the serum of the INF and the EN and iv) recognition of specific antigens in extracts of L3 detected by Western blot analysis. The specific immune response directed against L3 is probably a significant immunological defense mechanism used to limit infection in endemic areas.