Transcripts encoding discrete, cathepsin L-like cysteine proteases, known as SmCL1 and SmCL2, have been reported from the adult stages of the human blood fluke Schistosoma mansoni. However, the physiological roles of these 2 enzymes and their natural substrates remain uncertain and controversial. To determine their localization in adult S. mansoni by immunocytochemical procedures, and thereby to gain insight into their likely functions, polymerase chain reaction-based cDNAs encoding mature SmCL1 and SmCL2 were ligated into Escherichia coli. The bacterially expressed recombinant proteins (bSmSL1, bSmCL2) were used to generate monospecific rabbit antisera. For light microscopy, paraffin-embedded sections were visualized with the fluorophore Cy3. For transmission electron microscopy (TEM), LR White-embedded tissue was visualized with 15 nm gold. Under light microscopy, fluorescence was visible on the luminal surface of the gastrodermis in both sexes for both proteins. For bSmCL1 and bSmCL2, TEM revealed gold particles primarily associated with amorphous deposits within superficial digestive vacuoles on the gastrodermal surface of males and females. Some bSmCL1 reaction product was observed in vesicles within the gastrodermis, and very sparse gastrodermal activity was observed with bSmCL2. By contrast, neither enzyme was immunolocalized in the reproductive organs, vitelline glands, nor gynecorphoric canal. The gut-associated immunolocalization of SmCL1 and SmCL2 indicates that both these endopeptidases participate in hemoglobin proteolysis.

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