Immunocytochemical localization of a cysteine protease in adult worms of the liver fluke Fasciola sp.

The intracellular localization of a cysteine protease from Fasciola sp. that hydrolyzes host hemoglobin as a nutritional source was examined by light and electron microscopic immunocytochemistry using a monoclonal antibody specific to the enzyme. Immunoperoxidase staining was predominantly restricted to large numbers of granules in the parasite intestinal epithelial cells and to host erythrocytes present in the intestinal lumens. In immunogold electron microscopy, the gold particles were consistently deposited on the electron-dense secretory granules in intestinal epithelial cells and on the intestinal contents. These findings suggest that the Fasciola cysteine protease in the secretory granules is secreted as a digestive enzyme into the intestinal lumen, where it may play an important role in the extracellular degradation of host proteins, including hemoglobin.