Exploring the role of intestinal barrier in swine enteric coronaviruses infection

Swine enteric coronaviruses (SeCoVs) infection pose a significant threat to the global swine industry, manifesting as symptoms such as vomiting, watery diarrhea, severe dehydration, and high mortality rates, especially among neonatal piglets. The intestinal barrier serves as a critical defense against intestinal microorganisms and plays a pivotal role in maintaining host homeostasis. This barrier comprises the mucus layer, intestinal epithelium, intestinal microbiota and underlying immune cells. However, the role of the intestinal barrier in countering SeCoVs infection and its influence on viral susceptibility remain poorly understood. In addition, the inherent limitations of existing cell models further complicate the study of the interaction between SeCoVs and the intestinal barrier. In the first part of our study, we investigated the factors contributing to the heightened susceptibility of neonatal piglets to SeCoVs infection, with a focus on the intestinal barrier. Our findings reveal that one-week-old piglets exhibit longer intestinal villi with reduced crypt depth, underdeveloped epithelial cells, a lower presence of probiotic bacteria, and an immature immune system compared to older pigs. Furthermore, one-week-old piglets show elevated levels of proteins that assist enteric virus invasion but lower levels of resistance proteins. These factors collectively provide a reasonable explanation for SeCoVs susceptibility in neonatal piglets. The second part of our research explores the impact of the intestinal epithelium on SeCoVs invasion. We developed a porcine intestinal organoid monolayer to investigate the interaction of transmissible gastroenteritis virus (TGEV) with the intestinal epithelium. Our results demonstrate that TGEV infection induces interferon (IFN) and inflammatory responses in this organoid model, but it does not disturb the integrity of the intestinal epithelium. Further investigation revealed that TGEV infection stimulates the proliferation of intestinal stem cells (ISCs) through the activation of the Wnt/β-catenin pathway, promoting the self-renewal of epithelial cells and thereby preserving epithelial integrity. The third part of our study focuses on the role of the mucus layer in SeCoVs invasion. We developed a porcine intestinal organoid monolayer with a mucus layer to investigate SeCoVs infection within this context. Our results indicate that the mucus layer acts as a protective barrier against porcine epidemic diarrhea virus (PEDV) and TGEV infections. Further investigation revealed that mucin 2 (Muc2) exerts antiviral activity in this infection scenario. Additionally, neuraminidase (NA) treatment effectively alleviates the inhibitory effect of Muc2 on SeCoVs infection, highlighting the role of sialic acid modification of Muc2 in its antiviral activity. In conclusion, this thesis provides an in-depth analysis of the factors contributing to the high susceptibility of newborn piglets to SeCoVs from the perspective of the intestinal barrier. It also explores the interactions between SeCoVs and the intestinal epithelium and mucus layer through the development of an intestinal organoid system. These findings offer a comprehensive understanding of the role of the intestinal barrier in SeCoVs infection and provide valuable insights for the development of prevention and control strategies against SeCoVs.