Schistosoma mansoni: localisation of antigenic regions on the 31 kilodalton diagnostic protein.

Antigenic sites on the 31 kDa diagnostic protein of Schistosoma mansoni (Sm31) were identified using the cDNA fragment H3 cloned in the expression vector pEx34b. This fragment encodes approximately two-thirds of the polypeptide. A set of deletion mutants was generated by the exonuclease Bal31 and the resulting shortened proteins synthesised in Escherichia coli as fusions to MS2 polymerase were tested in Western blots with human schistosomiasis patient sera. Three antigenic regions were identified, one of which was narrowed down by appropriate restriction sites to a sequence specifying only 27 amino acid residues. Examination of a longer MS2 fusion product extending into the N-terminus of the protein, corresponding to the nearly full length Sm31 sequence, revealed that its reactivity in immunoblots is comparable with that of the H3 clone. This suggests that additional antigenic sites, which might be more reactive than those already identified, are absent from the remaining part of the protein.