• celiac disease;
  • refractory celiac disease;
  • epithelial barrier;
  • tight junction

In celiac disease, the gut-associated immune system is activated in response to the ingestion of gluten, causing an atrophy of the small intestinal mucosa. Although this condition is, in most cases, responsive to a gluten-free diet, celiac disease refractory to treatment occurs in a small percentage of celiacs. An epithelial barrier defect is known to be an integral part of celiac pathophysiology. However, the mucosa in refractory celiac disease underlies a constant inflammatory process. The epithelial barrier has not been addressed in this condition so far. Herein, the tight junction-associated barrier in refractory celiac disease is investigated functionally and structurally. Although normally expressed in celiac disease, claudin-4 is shown to be downregulated in refractory cases, presumably by two mechanisms, reduced protein expression and increased claudin endocytosis. Furthermore, the tightening claudin-5 is downregulated and the pore-forming claudin-2 is upregulated.