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In the present study we describe type 1 cystatin, a cysteine protease inhibitor, as a
major released antigen of the tropical liver fluke Fasciola gigantica (FgStefin-1).
Immunohistochemical analysis showed that FgStefin-1 is abundant in tissue of tegumental
type and the intestinal epithelium. Faint staining was observed in the epithelia of ovary and
proximal uterus. Immunoblots showed the presence of FgStefin-1 in the parasite’s
excretion/secretion (ES) product. Sera of experimentally infected sheep reacted with
recombinant FgStefin-1 starting 8 weeks postinfection. Activity analyses of recombinant
FgStefin-1 showed nanomolar inhibition constants for mammalian host and parasite cysteine
proteases. Our results suggest protective functions of FgStefin-1, regulating intracellular
cysteine protease activity, and possibly protection against extracellular proteolytic damage to
the parasite’s intestinal and tegumental surface proteins.
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