Figure 2.

Paracellular intestinal barrier in health and inflammation. The paracellular barrier in healthy distal intestinal tissue is characterized by high expression levels of barrier-forming TJ proteins, which leads to a low paracellular permeability. In inflammation, the TJ barrier is disturbed. Channel-forming claudins are up-regulated in their expression (eg, by TNF-α or IL13), leading to an increased permeability for ions and water (pore pathway). Barrier-forming TJ proteins are down-regulated and can also be shifted into subjunctional regions or into endosomes, which further destabilizes the TJ barrier. Occludin, which is, besides tricellulin, involved in regulation of permeability for macromolecules, is also down-regulated by inflammatory processes (eg, by TNF-α or interferon-γ), leading to increased paracellular permeability for macromolecules (leak-pathway). IFN, interferon.