IDENTIFICATION AND CHARACTERIZATION OF THE 'GUT VASCULAR BARRIER'

In order to protect the body from a wide range of harmful environmental agents, the intestine has developed a number of barrier mechanisms to limit the entry of potential hazards. These include the physical barrier formed by the epithelial layer and the intestinal immune system that is important to induce either tolerance against food antigens and intestinal flora or inflammatory responses against dangerous microorganisms. It has been demonstrated that tolerance against commensal bacteria is strictly compartmentalized, in the sense that the systemic immune system is completely unprimed by these bacteria. It was demonstrated that the mLNs function as a “firewall” confining induction of tolerance to the mucosa while the systemic immune system remains ignorant to these bacteria. However, in these studies how the bacterial flora is excluded from the entrance in the bloodstream via the intestinal blood vessels has not been analyzed. Here, we describe a new barrier that we called the GVB (gut vascular barrier) that plays a fundamental role in controlling the spreading of molecules and bacteria to systemic sites. We found that intestinal endothelial cells (ECs) express the main components of TJs (occludin, JAM-A, CLDN-12, ZO-1 and cingulin) and AJs (VE-cadherin and junctional β-catenin), indicating the presence of a barrier that excludes bacteria from passing through the paracellular route. In addition, we observed the existence of a “gut vascular unit” (GVU) whereby ECs were associated with enteric glial cells and pericytes, whose role in the establishment of the endothelial barrier phenotype remains to be analyzed. Moreover, we show that GVB integrity could be modified by Salmonella typhimurium infection. Indeed, upon infection ECs up-regulated the expression of PLVAP, that has been previously used as a marker of immature/damaged vascular barrier in the brain, and up-regulated caveolin-1, the major component of caveolae. These changes correlated with a higher permeability of the endothelium to small molecules and to bacteria. One way by which S. typhimurium could modify the barrier properties of the intestinal blood vessels could be through the negative regulation of the Wnt/β-catenin signaling pathway. Indeed, we found that the activation of β-catenin was reduced upon Salmonella infection in vitro. Consistently, we found that Salmonella was incapable to modify ECs permeability and to spread systemically in mice where β-catenin was constitutively activated by genetic means only in vascular ECs. Furthermore, it appeared that the TTSS encoded by Salmonella pathogenicity island-2 was involved in the regulation of Wnt/β-catenin signaling pathway in ECs. Finally, preliminary results show that the microbiota could induce GVB maturation and maintenance. However, the mechanisms involved in these processes as well as the bacterial species responsible for this process have not been investigated yet.