The interplay between Giardia intestinalis and host intestinal epithelial cells : A tale of immune activation and suppression

Sammanfattning: Giardia intestinalis is a non-invasive, protozoan parasite causing the diarrheal disease giardiasis in a variety of mammals, including humans. The parasite has two main life cycle stages; disease-causing trophozoites and infectious cysts. Trophozoites colonize the host’s upper small intestine by attaching to intestinal epithelial cells (IECs). G. intestinalis infections can cause a broad spectrum of clinical outcomes, ranging from acute and chronic infections to asymptomatic carriage. In most cases, the infection causes only a low degree of mucosal inflammation. The molecular basis of how the parasite causes such diverse clinical outcomes is poorly defined.  Here, we used in vitro infection models to investigate host-parasite interactions between G. intestinalis and IECs. In paper I, we assessed transcriptional changes of Caco-2 cells during infections with different life cycle stages of the parasite by RNA-sequencing (RNA-seq). Infections with trophozoites, cysts or encysting cells showed that each life cycle stage of the parasite induced the expression of both a common set of host genes and unique, life cycle stage specific responses. The IECs showed large expression changes in immune signalling, transcriptional regulation, apoptosis, metabolism and oxidative stress. We could also observe that the parasites’ contact with IECs inhibited its encystation processes. In paper II, we mapped global protein changes of Caco-2 cells during G. intestinalis trophozoite infections using quantitative proteomics. We could confirm expression changes in oxidative stress, immune response, signalling, metabolism and apoptosis on protein level. We further investigated the interplay between IECs and G. intestinalis using small intestinal organoids, to more accurately mimic intact primary host tissue. The infection dynamics of trophozoites on intestinal organoid-derived monolayers were assessed using live-cell microscopy (paper III). We investigated swimming behaviour of trophozoites on the intestinal epithelial surface. Trophozoites swan in repetitive circling and intermittent attachment patterns during preattachment swimming, possibly to explore the epithelium surface and to find a suitable permanent attachment site. We further assessed early immune responses of IECs during trophozoite infections using intestinal organoid-derived monolayers (paper IV). Transcriptional expression of immune response genes was dependent on the trophozoite inoculate. Trophozoites that were preconditioned to maximize the trophozoites’ fitness triggered only a negligible inflammatory transcriptional response, while lysed and “non-fit” trophozoites induced a strong inflammatory response. “Fit” trophozoites could even suppress the inflammatory response triggered by lysed trophozoites, demonstrating the immune modulating capacity of the parasite. In conclusion, this thesis has increased our understanding of the interplay between G. intestinalis and IECs.