Characterization of a stage-specific M r16 000 schistosomular surface glycoprotein antigen of Schistosoma mansoni

Abstract

A 16 kDa Schistosoma mansoni schistosomular surface antigen (Sm16) was originally described as the target of a passively protective mAb (B3A). It appeared on the schistosomular surface after transformation of cercariae and was uniquely recognised by sera from animals exposed to attenuated cercariae. In this work sequential extractions of schistosomula with Triton X-114 and sodium dodecyl sulphate showed Sm16 to be an integral membrane structure which did not appear to be glycosylphosphatidylinositol-anchored as judged by experiments using phosphatidyl inositol-specific phospholipase C. The antigen was strongly reactive in Western blotting with rabbit irradiated vaccine sera. Sm16 was demonstrated in the hepatopancreas of S. mansoni-infected snails and was equally abundant in cercariae and mechanically- transformed schistosomula but was undetected in liver stage worms or eggs. Immunoelectron microscopy showed Sm16 to be localised, in cercariae, to what are believed to be subtegumental cell bodies packed with membraneous vesicles. Treatment with proteases and with sodium metaperiodate showed Sm16 to be a glycoprotein of which the epitope recognised by B3A was periodate sensitive. Two-dimensional electrophoresis gave a PI of 6. Neither the size or the recognition by B3A was affected by treatment with N-glycosidase F, endoglycosidase F or endo-α- N-acetylgalactosaminidase. Western blotting using a wide range of biotinylated lectins showed recognition only by peanut agglutinin and Ricinus communis agglutinin II (ricin). It is concluded that Sm16 has antigenic surface-exposed O-linked complex oligosaccharides which lack mannose/glucose, GlcNAc, L-fucose and sialic acid but contain terminal Gal β (1–3) GalNAc and/or galactose.