Schistosoma mansoni: evidence for a role of serum factors in protecting artificially transformed schistosomula against antibody-mediated killing in vitro.

Artificially transformed schistosomula of Schistosoma mansoni develop a consistent but small protection against the lethal effects of antibody plus complement when cultured for 24 h in a chemically defined medium. In contrast, they become rapidly resistant to antibody plus complement, when cultured in the presence of a complex medium consisting of equal parts of heat-inactivated rabbit serum and Earle's/lactalbumin or in defined medium supplemented with small amounts of heat-inactivated rabbit serum. Sephadex G-200 gel filtration revealed that the protective factor in rabbit serum is a macromolecule with a molecular weight between 7 and 19 S. Parasites cultured at 10 degrees C or in the presence of 200 microgram of puromycin show less serum-induced protection against the lethal effects of antibody plus complement than do controls.