Date of Award
Doctor of Philosophy
The protozoan parasite, Giardia lamblia (syn. G. intestinalis, G. duodenalis), is a leading cause of pathogen-induced gastrointestinal disease throughout the world. Upon Giardia colonization of the proximal small intestine, epithelial barrier dysfunction is induced through enterocyte apoptosis and disruption of the tight-junction protein complexes. Intestinal barrier disruption promotes the translocation of luminal contents, including commensal bacteria and Giardia-derived excretory-secretory products (ESPs), into the mucosal tissue. In an attempt to elucidate the cellular mechanisms of giardiasis, we constructed a novel in vitro co-culture model of epithelial cells (Caco-2 cells) and macrophages (IC-21 cells) for evaluating long-term host-parasite interactions. We show that epithelial cells secrete unique cytokines when co-cultured with macrophages and that Giardia is capable of abolishing these pro-inflammatory cytokines. We employed the developed model to conduct a longitudinal assessment of Giardia-induced barrier dysfunction. Our data indicate that enterocyte apoptosis, transepithelial resistance, and localization of the the tight-junction protein, ZO-1, are impacted by 5 days of parasite interaction. Secondary to the breakdown in barrier function, our studies highlight that Giardia ESPs that invade the lamina propria tissue are capable of suppressing bacterial-activated inflammatory signaling cascades in macrophages. The modulation of toll-like receptor (TLR) signaling occurs through TLR4-dependent decreased activation of IL-1 receptor associated kinase-4 (IRAK-4). Additionally, there is reduced activity of E. coli stimulated pro-inflammatory mitogen-activated protein kinases (MAPKs), p38 and JNK, in macrophages exposed to Giardia conditioned medium. Our findings indicate that Giardia can inhibit mobilization of the immune response by regulating macrophage signaling cascades. Collectively, these data suggest that Giardia-induced barrier dysfunction may increase the pathogenesis of the parasite by promoting the translocation of immune-suppressive components into the host, thereby increasing the prospect of infection persistence.
dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.
Fisher, Bridget Sutton, "Modeling Giardia-Host Interactions: Mechanisms of Barrier Dysfunction and Immune Suppression" (2014). Electronic Theses and Dissertations. 1011.