Comparison of the role of complement in immunity to Schistosoma mansoni in rats and mice.

In vivo depletion of C3 with cobra venom factor (CoF) was used to demonstrate the participation of complement in the innate immunity to S. mansoni and in the acquired immunity of both actively and passively immunized rats. Complement was shown to play an important role in innate immunity, being more involved later in larval migration (Days 8-13 post-infection) than at earlier times (Days--1-3 and Days 3-8 post-infection). Furthermore, the specific component of immunity conferred by immune serum transferred at the lung-migration stage also required complement for optimal expression. This supports the notion that both innate and acquired immunity act not against the much studied early post-penetration stages, but primarily against the lung stages. Although decomplementation at earlier stages of parasite migration (up to 3 days post-infection) did cause some reduction of innate immunity, there was no evidence of any effect on the levels of resistance actively induced by exposure to irradiated cercariae. This suggests that, while complement may play a role in innate immunity during the skin-migration phase, specific complement-mediated attrition does not play a crucial role at this time. The situation was very different in the mouse model, since no involvement of complement in either innate or irradiated vaccine-induced immunity could be demonstrated within the first 15 days of infection. Thus, there appear to be phases in the parasite migration in rats, but not in mice, during which complement becomes a critical factor in both innate and acquired immunity to S. mansoni.