Human monocyte-derived macrophages are lysed by schistosomula of Schistosoma mansoni and fail to kill the parasite after activation with interferon gamma.

In this study was examined the interaction between schistosomula of Schistosoma mansoni and human monocyte-derived macrophages activated with interferon gamma (IFN-gamma). Peripheral blood monocytes were matured for 6 days and activated by further culture with IFN-gamma (600 U/ml). These IFN-gamma-treated monocyte-derived macrophages are cytotoxic for the tumor cell line K562, which is not killed by nonactivated monocyte-derived macrophages. Activated monocyte-derived macrophages were incubated with schistosomula at ratios of 10(3):1 and 10(4):1 in the presence of serum pooled from patients with schistosomiasis. This antiserum promoted an increased adherence of cells to the parasite. However, the activated monocyte-derived macrophages failed to kill the schistosomula under all conditions tested. On the contrary, the monocyte-derived macrophages were killed by schistosomula in a time-dependent and antibody-dependent manner, which was most evident at a lower effector/target ratio, 200:1. Electron microscopy showed that monocyte-derived macrophages were lysed on the surface of schistosomula. Further, both monocyte-derived macrophages and contaminating blood platelets fused with the parasite surface membrane, so that the cell plasma membrane and the outer tegumental membrane formed a hybrid membrane. The results indicate that matured human monocyte-derived macrophages activated by IFN-gamma are unable to kill schistosomula. Instead, the effector cells fuse with the parasites and are lysed by them.