The protective role of acquired host antigens during schistosome maturation.

Schistosomes grown in mice were tested at different stages of development for susceptibility to an in vitro cytotoxic effector mechanism involving eosinophils and an antibody directed against mouse determinants. Despite the fact that 5-day lung worms and 6-week adult worms both bound the antibody to their surfaces, eosinophils attached preferentially to the adults and killed them. Complement had an enhancing effect in this system. Those eosinophils which did adhere to the lung worms degranulated onto the tegument but were unable to mediate damage or killing, even when complement was activated at the parasite surface. The resistance shown by the lung worms was shared by 2-week worms and small 3-week worms. Larger 3-week worms and older stages were, however, susceptible to cell-mediated cytotoxicity in this system. We suggest that the host antigen disguise constitutes the major protective mechanism utilized by older schistosomes to evade immunity, but that the younger stages have an additional and equally effective mechanism of resistance.