Partial characterization and kinetics of expression of Sm15, a Schistosoma mansoni tegumental antigen

Differential antibody screening of an adult Schistosoma mansoni cDNA expression library constructed in lambda gt11 identified a partial cDNA clone, A70. This cDNA encodes a fusion protein recognized by antibodies raised against highly irradiated schistosomula and adult worm tegumental membranes but not by anti-egg antibodies. Anti-tegumental membrane antisera affinity-purified on the A70 cDNA fusion protein were used for Western blotting analysis and indirect immunofluorescence, resulting in the identification of a 15-kDa protein (Sm15) in the tegument of adult worms. This is one of the principal tegumental antigens recognized by antibodies from mice protectively vaccinated with adult worm tegumental membranes. Sm15 is much smaller than the protein encoded by its gene, suggesting that it results from a highly processed precursor. It was found that Sm15 behaves as an integral membrane protein upon partitioning in Triton X-114 and that it is present in worms of 2 weeks or older but not in schistosomula or miracidia. The affinity-purified antibodies also revealed the presence of a 23-kDa antigen in whole-worm homogenates that is apparently coexpressed with Sm15. The 23-kDa antigen was not found associated with membranes and is probably a soluble protein. A further series of Western blots were undertaken using antibodies affinity-purified from serum raised against schistosomula. In this case, the 23- and 15-kDa products were not recognized, but rather soluble proteins ranging from 45- to 150-kDa were detected in almost all larval stages investigated. The results suggest that the precursor is differentially processed during maturation.