Protease secretion by Giardia duodenalis trophozoites upon interaction with epithelial cells and its association with the parasite adhesion was studied in co-cultures of parasites with IEC6 epithelial cell monolayers in the presence or absence of protease inhibitors. Proteolytic activity in supernatants from trophozoites was enhanced when they were co-cultured with IEC6 cells. This activity was strongly inhibited by pre-incubation of live trophozoites with E-64 and TPCK and a concomitant inhibition of parasite adhesion to IEC6 cells was observed. These data suggest that trophozoites secrete cysteine-type proteases that play a role in the adhesion of G. duodenalis to epithelial cells.Key words: Giardia duodenalis -protease secretion -adhesion Giardiasis caused by the flagellated, intestinal protozoan parasite Giardia duodenalis (syn. G. lamblia, G. intestinalis) manifests as acute and chronic diarrhoea in communities of both developing and developed countries. It is also a leading cause of defined waterborne diarrhoea worldwide (Adam 1991). Infection results from ingestion of cysts, most commonly from faecal contamination of water and food whereas community-wide outbreaks result from contaminated water supplies. After excystation in the upper small intestine, adhesion of trophozoites to epithelial intestinal cells represents the first step in the pathogenesis of the disease. The use of IEC6, an epithelial cell line derived from rat small intestine, is a well validated model in which the inhibitory effect of both actin inhibitors and chelating agents on the attachment of G. duodenalis trophozoites has been previously demonstrated (McCabe et al. 1991). It is also well recognized that the adhesion process is a multi-factorial event that involves a suction force of the adhesive disk (Knaippe 1990), a mechanical process related to contractile proteins of the adhesive disk and the ventrolateral flange (Chávez et al. 1995) and surface molecules that allow the binding of trophozoites to receptors to host epithelial cells (Pegado & Souza 1994). It has been postulated that proteases may be involved in this process. Several studies have demonstrated protease activity in total extracts of Giardia trophozoites or in parasite vacuoles (Feely & Dyer 1987, Hare et al. 1989) including the Portland 1 (P1) strain cultured in vitro, with the predominance of cysteine, serine, and aspartic protease activities (Williams & Coombs 1995). It has also been shown that cysteine proteases of P1 strain degraded protein substrates such as gelatin, collagen, albumin, and azocasein while serine proteases were mainly active against haemoglobin (Coradi & Guimarães 2006). In addition, the presence of cysteine protease activities in excretory/secretory (E/S) products of P1 strain has also been reported (Jiménez et al. 2000). These E/S products are associated with mucosal injuries observed in murine giardiasis (Jiménez et al. 2004). However the possible relation between protease secretion and the adhesion of G. duodenalis to epithelial cells has not b...