Cytotoxic effects in vitro of human monocytes and macrophages on schistosomula of Schistosoma mansoni

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Abstract
Human peripheral blood monocytes from normal donors were isolated by differential centrifugation and cultured in vitro in hydrophobic Teflon-coated tissue culture bags. Cells were harvested between 0 and 10 days and tested for their ability to kill schistosomula of Schistosoma mansoni in an in-vitro cytotoxicity assay. Freshly isolated, unstimulated monocytes demonstrated minimal cytotoxic capability. However, this was increased if the cells were pretreated with human recombinant gamma interferon (IFN-.gamma.), or with specific anti-S. mansoni antiserum. As the monocytes matured in vitro there were marked increases in the levels of antibody-independent killing of schistosomula. Monocytes grown in vitro with IFN-.gamma. (104 u/ml) took 2-3 days to develop almost maximal cytotoxicity (mean 94% kill of schistosomula). In contrast, unstimulated monocytes (no IFN-.gamma.) took between 5 and 7 days to achieve comparable cytotoxicity (mean 99% kill). Killing of the schistosomula was dependent upon a high effector to target ratio, and was a relatively slow phenomenon in vitro, parasite attrition occurring between 17 and 36 h. Supernatants from cytotoxic macrophages were ineffective in mediating cytotoxicity of the parasite.