The
asparaginyl endopeptidase (
Sm32) is expressed in the gastrodermal cells of the schistosome gut and in the head glands of the cercariae. Possibly,
Sm32 hydrolyzes pro-
proteins involved in the degradation of host
hemoglobin [Parasitol. Today 12 (1996) 125]. Preliminary evidences using an Sj32/
Sm32 murine
vaccine have shown a profound effect on oviposition and worm burden [Chin. J. Schist. Control. 7 (1995) 72; Bull. Human Med. Univ. 24 (1999) 225;
Vaccine 20 (2002) 439]. The importance of
Sm32 as a novel
vaccine candidate is based on the possibility of preventing the maturation of other
cathepsins and/or preventing schistosome skin invasion. We studied the immunogenicity of polymerizable
peptides derived from
Sm32 to select potential protective
epitopes.
Sm32 prediction of T and B
epitopes and homology studies with human
legumain were performed. Among the variety of factors that influence the antibody response, we specifically examined the effect of: (i) genetic background of mouse strain, inbred (C57BL/6) versus outbred (Swiss) mice; and (ii) vaccination with a single
peptide versus pool of
peptides. Swiss mice raised
antibodies to three different regions of the
Sm32, as tested by the Multiple
Antigen Blot Assay (MABA): 182-215 (
peptides IMT-70 and 72), 244-273 (IMT-64) and 336-355 (IMT-66). None of these regions were immunogenic for C57BL/6. On the contrary, other
peptides, IMT-4 (21-40), IMT-12 (101-120) and IMT-26 (292-313) were highly immunogenic for this inbred strain. Only Swiss mice immunized with a single
peptide (IMT-64 and 72) or with three different pools of IMT-
peptides (Pool A-II: 14, 16, 18, 70, 72, 89; pool A-III: 22, 64, 24, 26, 28 and pool A-V: 64, 66, 28, 70, 72) recognized the original
protein in a
crude extract of the worm
antigen by Western blot.
Peptides IMT-64, 14 and 26 were responsible for this recognition. In general, the vaccination with pool of
peptides was more immunogenic for both mouse strains. Predicted
B cell epitopes, with hydrophilicity scores over +10 (IMT-12, 64, 26) were always immunogenic after either single or combined
peptide vaccination.
Sm32 sequences 41-80 (IMT-6 and 8), 141-160 (IMT-16) and 182-215 (IMT-70 and 72) were nearly identical to the corresponding human
legumain regions and should be excluded from the human
vaccine. We can conclude that the regions of
Sm32 that were recognized by
antibodies of mice immunized with polymerizable
peptides depended on the mice strain and on the hydrophilicity score of the
peptides.