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Detection of the Sm31 antigen in sera of Schistosoma mansoni– infected patients from a low endemic area

Schistosoma mansoni cathepsin B (Sm31) is a major antigen from adult worms that circulates in the blood of infected patients (Li et al., Parasitol Res 1996; 82: 14–18). An analysis of the Sm31 sequence (Klinkert et al., Mol Biochem Parasitol 1989; 33: 113–122) allowed the prediction of seven hydrophilic regions that were confirmed to be exposed on the surface of a 3D model of Sm31; the species specificity of these regions was checked using BLAST analysis. The corresponding peptides were chemically synthesized in polymerazible forms using the t‐Boc technique. Rabbits developed a high humoral response against these peptides as tested by a multiple antigen blot assay; it recognized native Sm31 in crude S. mansoni extracts and as circulating antigen in sera of S. mansoni‐infected patients by western blot. Relevant antigenic determinants were located at the N‐ and C‐terminus sequences. Antibodies against these regions recognized the native enzyme in an ELISA‐like assay called cysteine protease immuno assay in which the immunocaptured enzyme was revealed by the intrinsic cathepsin B hydrolytic activity of Sm31. The method successfully and specifically detected Sm31 in sera of infected individuals, most of them (83·3%) with light infections, offering a rationale for the development of parasite enzyme capture assays using anti‐synthetic peptide antibodies for possible use in the diagnosis of schistoso,iasis.