Tight junction regulation, intestinal permeability, and mucosal immunity in gastrointestinal health and disease

Purpose of review The contributions of intestinal barrier loss, that is, increased permeability, to multiple disorders, including inflammatory bowel disease (IBD), have been a topic of speculation for many years, and the literature is replete with conclusions based on correlation and speculation. The goal of this article is to critically review recent advances in mechanistic understanding of barrier regulation and the evidence for and against contributions of intestinal barrier loss to disease pathogenesis. Recent findings It is now recognized that intestinal permeability reflects the combined effects of two distinct routes across tight junctions, which form selectively permeable seals between adjacent epithelial cells, and mucosal damage that leads to nonselective barrier loss. These are referred to as pore and leak pathways across the tight junction and an unrestricted pathway at sites of damage. Despite advances in phenotypic and mechanistic characterization of three distinct permeability pathways, development of experimental agents that specifically target these pathways, and remarkable efficacy in preclinical models, pathway-targeted therapies have not been tested in human subjects. Summary After decades of speculation, therapeutic interventions that target the intestinal barrier are nearly within reach. More widespread use of available tools and development of new tools that discriminate between pore, leak, and unrestricted pathway permeabilities and underlying regulatory mechanisms will be essential to understanding the local and systemic consequences of intestinal barrier loss.