Prevention of SARS-CoV-2 entry into cells through the modulation of viral host receptors, such as ACE2, could represent a new chemoprophylactic and therapeutic approach for COVID- 19 complementing vaccination. However, the mechanisms controlling ACE2 expression remain elusive, partly due to low ACE2 levels in experimental models. To address this challenge, I use biliary epithelial cell (cholangiocyte), one of the cell types with the highest ACE2 levels, to dissect the molecular mechanisms controlling ACE2 expression. I identify the farnesoid X receptor (FXR) as a direct regulator of ACE2 transcription in multiple COVID19- affected tissues, including the gastrointestinal and respiratory systems. I use this knowledge to optimise current experimental models for SARS-CoV-2 infection and I demonstrate that reduction of FXR signalling, with the over-the-counter compound z-guggulsterone (ZGG) and the off-patent drug ursodeoxycholic acid (UDCA), downregulates ACE2 in human lung, cholangiocyte and intestinal organoids and in the corresponding tissues in mice and hamsters. I also show that UDCA-mediated ACE2 downregulation reduces susceptibility to SARS-CoV- 2 infection in vitro, in vivo and in human lungs and livers perfused ex situ. I then illustrate that UDCA reduces ACE2 expression in the nasal epithelium in humans. Finally, I present a retrospective analysis using an independent patient cohort which identify a correlation between UDCA treatment and positive clinical outcomes following SARS-CoV-2 infection, including hospitalisation, ICU admission and death using retrospective COVID-19 registry data, and confirm these findings in a second independent cohort of liver transplant recipients. In conclusion, with the work described in this dissertation I identify a novel function of FXR in controlling ACE2 expression and provide evidence that modulation of this pathway could be beneficial for reducing SARS-CoV-2 infection, thereby paving the road for future clinical trials.