Schistosoma mansoni: interactions of adult parasites with the complement system

The interaction of the mouse complement system with adult male Schistosoma mansoni was studied by immunocytochemical localization procedures and in-vitro assays for complement mediated tegument damage. Mouse C3 was demonstrated to be associated with the parasite's tegument, but was localized only in the infoldings of the tegument and not on its free surface. Freshly harvested parasites manifested no detectable tegumental modification when incubated in normal mouse serum or in immune mouse serum. However, parasites which had been allowed to lose their adsorbed host components by elution (incubation in serum free media for 3 h at 37 degrees C) were severely damaged by incubation in normal mouse serum, but not by incubation in immune mouse serum. This damage was shown to be mediated by the alternative complement pathway and appeared to be initially limited to the tubercles of the adult male parasite. Tegument disruption could be blocked by pre-incubation of the eluted worms in either immune mouse serum or an IgG fraction of immune mouse serum. An IgG fraction of normal mouse serum did not protect the parasite, and infected mouse serum (IMS) which had been depleted of IgG produced tegument damage equivalent to that observed with normal mouse serum (NMS). The addition of I-IgG to NMS abrogated tegument damage. These data suggest that while adult schistosomes possess surface molecules bearing alternative pathway complement activation sites, these sites are masked by adsorbed host components in vivo. These results further indicate that in the absence of these masking host molecules anti-schistosome IgG may play a role in protecting the adult worm from alternative pathway activation, perhaps by binding to and blocking the activation sites on the tegument associated molecules.