The resistance of schistosomula of Schistosoma mansoni to antibodies and complement in vitro does not correlate with the binding of antibodies to the surface of the parasite

Three-hour mechanical schistosomula of Schistosoma mansoni were surface labeled with dinitrophenyl (DNP) groups by short incubation with liposomes made of N-DNP-ε-aminocaproyl-phosphatidylethanolamine (DNP-Cap-PE)-egg lecithin (1:5). The lipid-conjugated DNP groups persisted on the surface of the worms, accessible to antibodies, for more than 48 hr under in vitro culture conditions, in defined synthetic medium (DSM) or in the presence of 50% normal human serum. Binding of rabbit anti-DNP and mouse monoclonal IgG1 anti-DNP to the surface of the labeled schistosomula was studied using fluorescent or ¹²⁵I-labeled antibodies or protein A. In DSM alone the bound antibodies were found to be evenly distributed over the entire surface of the worms. The presence of serum did not result in masking of surface antigens, nor did it decrease the extent of antibodies bound. Serum, however, induced a patchy pattern of binding that could be observed on 21-hr schistosomula. This pattern was much more prominent on 48-hr worms. The density of the DNP haptenic groups and their location in the outer membrane of 3-hr schistosomula rendered the worms susceptible to anti-DNP antibodies and complement. The labeled schistosomula, however, became resistant to this cytotoxic activity within 6 hr in DSM and within 2 hr in the presence of 50% human serum. This resistance of the parasite was found not to correlate with the ability of antibodies to bind, which did not significantly decrease during this time. Similar results were obtained with anti-schistosome antibodies from chronically infected mice. In the presence of 50% human serum, 3-hr mechanical schistosomula became resistant to these antibodies and to guinea pig complement within 6 to 8 hr, while the binding of the antibodies did not significantly decrease during this time. The results indicate that the insusceptibility to antibodies and complement that schistosomula develop within few hours after transformation does not result from decreased binding of the antibodies but from another, yet unknown, mechanism.