The host-parasite interface in molluscan schistosomiasis: biotin as a probe for sporocyst and hemocyte surface peptides.

Hemocytes of Biomphalaria glabrata mediate the internal defensive response which, in resistant snail strains, kills sporocysts of Schistosoma mansoni. Lacking a gut, the sporocyst has only its tegument to interact with the host milieu (hemolymph). We have, therefore, focused our study on the surface-exposed proteins of hemocytes and sporocyst tegument. Using gentle biotinylation of living systems, labelled proteins were studied after SDS-PAGE electrophoresis and transfer to nitrocellulose. Results validate the utility of surface biotinylation in studies of host and parasite interfaces. A low diversity characterizes hemocyte surfaces and strain-specific differences are not in evidence. Hemocyte surfaces differ distinctly from the plasma in which these cells reside. In contrast, sporocyst surfaces expose a wide variety of peptides. These are remarkably stable even when sporocysts procured in snail plasma-free media are exposed to plasma. Thus, antigenic differences seen previously when Western immunoblotting was used to study sporocyst surfaces appear to be manifestations of minor changes in the exposed peptides or changes not detectable with this methodology. Hemoglobin, acquired by sporocysts from snail plasma, is processed and disappears from the surface during an overnight chase in culture medium.