Macrophages as effector cells of protective immunity in murine schistosomiasis. II. Killing of newly transformed schistosomula in vitro by macrophages activated as a consequence of Schistosoma mansoni infection.

Plastic adherent peritoneal cells from Schistosoma mansoni-infected mice have previously been shown to exhibit nonspecific tumoricidal activity in vitro. In this report we show that these same cell populations kill significant numbers of skin-stage schistosomula in vitro in the absence of added antibody. Larval killing by these activated cells could be enhanced by the use of suspension rather than monolayer cultures and by addition of heat-inactivated immune mouse serum to the cultures. Adherence of cells to schistosomula was also enhanced under the same conditions, suggesting that cell binding to the larvae might be critical in the development or expression of microbicidal activity. In support of this hypothesis, the same level of enhancement of cell binding and larval damage was observed upon substitution of concanavalin A for immune mouse serum. Killing of schistosomula appeared to be mediated solely by activated macrophages in the peritoneal cell suspensions from S. mansoni-infected mice, because partially purified preparations of eosinophils were virtually inactive in these assays. Likewise, inflammatory macrophages from uninfected mice were unable to kill schistosomula under the same conditions, emphasizing the importance of activation in the development of killing capability. The finding that macrophages activated as a consequence of S. mansoni infection are able to kill larval schistosomes in vitro suggests that these cells may play a role in concomitant immunity to schistosomiasis in vivo.