Inhibition of cysteine proteinase from Schistosoma mansoni larvae by alpha-macroglobulin from the plasma of Biomphalaria glabrata.

The hemolymph of Biomphalaria glabrata, a molluscan host of Schistosoma mansoni, contains an alpha-macroglobulin proteinase inhibitor (alphaM). In this study we have demonstrated that this host molecule inhibits a cysteine proteinase produced by larval S. mansoni. Inhibition by alphaM involves conformational changes through proteolytic cleavage by the proteinase, thus the enzyme must be active for interactions to occur. A specific cysteine proteinase inhibitor (E64) was used to block the interaction between parasite cysteine proteinase and host alphaM during an in vitro parasite killing assay. Increased sporocyst mortality was not observed in hemolymph from susceptible strains of B. glabrata when E64 was included, nor was there decreased killing in similarly treated hemolymph from a resistant strain. This suggests that the inhibition of this parasite proteinase by host alphaM is not involved in processes determining either resistance or susceptibility to this trematode.