Adherence ofGiardia lambliaTrophozoites to Int-407 Human Intestinal Cells

Abstract

Attachment of Giardia lamblia trophozoites to enterocytes is essential for colonization of the small intestine and is considered a prerequisite for parasite-induced enterocyte dysfunction and clinical disease. In this work, coincubation of Giardia with Int-407 cells, was used as an in vitro model to study the role of cytoskeleton and surface lectins involved in the attachment of the parasite. This interaction was also studied by scanning and transmission electron microscopy. Adherence was dependent on temperature and was maximal at 37°C. It was reduced by 2.5 mM colchicine (57%), mebendazole (10 g/ml) (59%), 100 mM glucose (26%), 100 mM mannose (22%), 40 mM mannose-6-phosphate (18%), and concanavalin A (100 g/ml) (21%). No significant modification was observed when Giardia was pretreated with cytochalasins B and D and with EDTA. Giardia attachment was also diminished by preincubating Int-407 cells with cytochalasin B and D (5 g/ml) (16%) and by glutaraldehyde fixation of intestinal cells and of G. lamblia trophozoites (72 and 100%, respectively). Ultrastructural studies showed that Giardia attaches to the Int-407 monolayer predominantly by its ventral surface. Int-407 cells contact trophozoites with elongated microvilli, and both trophozoite imprints and interactions of Giardia flagella with intestinal cells were also observed. Transmission electron microscopy showed that Giardia lateral crest and ventrolateral flange were important structures in the adherence process. Our results suggest a combination of mechanical and hydrodynamic forces in trophozoite attachment; surface lectins also seem to mediate binding and may be involved in specific recognition of host cells.Giardia lamblia, a parasitic flagellated protozoan, is the most common causative agent of diarrheal illness worldwide. In spite of significant recent advances in the knowledge on the biochemistry and molecular biology of G. lamblia, little is known about the pathogenesis of symptomatic infections in humans and the factors that determine the variability of the clinical outcome. A combination of parasitic factors and host responses seems to be involved, but damage of the intestinal epithelium by adherent trophozoites of G. lamblia has been proposed as one important mechanism in the pathogenesis of the infection (21). The structural modifications produced by G. lamblia trophozoites on epithelial cells are the result of close attachment of a contractile region of the ventral disk (30).The mechanism of attachment of trophozoites to intestinal cells has not been established definitively. Evidence supports roles for the ventral disk, which is considered a specific attachment organelle (19), trophozoite contractile elements (12), hydrodynamic and mechanical forces (20), and lectin-mediated binding (8, 26). However, experimental verification has been hindered by the lack of a suitable model. Previous studies of adherence have used a variety of model systems, including synthetic surfaces such as plastic and glass, nonhuman cells such as isolated rat enterocyt...

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