BACKGROUND & AIMS Tumor necrosis factor (TNF) increases intestinal epithelial cell shedding and apoptosis, potentially challenging the barrier between the gastrointestinal lumen and internal tissues. We investigated the mechanism of tight junction remodeling and barrier maintenance, as well as the roles of cytoskeletal regulatory molecules during TNF-induced shedding. METHODS We studied wild-type and transgenic mice that express the fluorescent-tagged proteins enhanced green fluorescent protein–occludin or monomeric red fluorescent protein1–ZO-1. After injection of high doses of TNF (7.5µg, i.p.), laparotomies were performed and segments of small intestine were opened to visualize the mucosa by video confocal microscopy. Pharmacologic inhibitors and knockout mice were used to determine the roles of caspase activation, actomyosin, and microtubule remodeling and membrane trafficking in epithelial shedding. RESULTS Changes detected included redistribution of the tight junction proteins ZO-1 and occluding to lateral membranes of shedding cells. These proteins ultimately formed a funnel around the shedding cell that defined the site of barrier preservation. Claudins, E-cadherin, F-actin, myosin II, Rho-associated kinase (ROCK), and myosin light chain kinase (MLCK) were also recruited to lateral membranes. Caspase activity, myosin motor activity, and microtubules were required to initiate shedding, whereas completion of the process required microfilament remodeling and ROCK, MLCK, and dynamin II activities. CONCLUSIONS Maintenance of the epithelial barrier during TNF-induced cell shedding is a complex process that involves integration of microtubules, microfilaments, and membrane traffic to remove apoptotic cells. This process is accompanied by redistribution of apical junctional complex proteins to form intercellular barriers between lateral membranes and maintain mucosal function.

γδ intraepithelial lymphocytes (IELs) are located beneath or between adjacent intestinal epithelial cells and are thought to contribute to homeostasis and disease pathogenesis. Using in vivo microscopy to image jejunal mucosa of GFP γδ T-cell transgenic mice, we discovered that γδ IELs migrate actively within the intraepithelial compartment and into the lamina propria. As a result, each γδ IEL contacts multiple epithelial cells. Occludin is concentrated at sites of γδ IEL/epithelial interaction, where it forms a ring surrounding the γδ IEL. In vitro analyses showed that occludin is expressed by epithelial and γδ T cells and that occludin derived from both cell types contributes to these rings and to γδ IEL migration within epithelial monolayers. In vivo TNF administration, which results in epithelial occludin endocytosis, reduces γδ IEL migration. Further in vivo analyses demonstrated that occludin KO γδ T cells are defective in both initial accumulation and migration within the intraepithelial compartment. These data challenge the paradigm that γδ IELs are stationary in the intestinal epithelium and demonstrate that γδ IELs migrate dynamically to make extensive contacts with epithelial cells. The identification of occludin as an essential factor in γδ IEL migration provides insight into the molecular regulation of γδ IEL/epithelial interactions.intestine | tight junction T he intestine is one of the few peripheral tissues to contain a large population of intraepithelial lymphocytes (IELs), with one IEL for every 5-10 epithelial cells. Although the majority of these IELs express the γδ T cell receptor, and epidermal γδ IELs have been studied extensively (1-4), the functions of intestinal γδ IELs remain poorly understood. Some studies have shown that γδ IELs contribute to progression of immune-mediated colitis (5-7); other data suggest that γδ IELs contribute to mucosal homeostasis (8, 9) by secreting keratinocyte growth factor (10, 11) and antimicrobial peptides (12, 13), suppressing CD4 + T-cell expansion through TGF-β and IL-10 production (8, 9) and promoting barrier maintenance via poorly understood mechanisms (13-15). These observations and the small number of IELs relative to intestinal epithelial cells are difficult to reconcile with the widely held belief that γδ IELs have limited motility (1, 16).Further understanding of γδ IEL function will require definition of the molecular structures that regulate interactions between intestinal epithelial and γδ T cells. On the basis of the location of epithelial/γδ IEL contact sites along epithelial lateral membranes, it is likely that epithelial proteins targeted to these domains, including apical junction complex components, are involved in these interactions. Attractive candidates include E-cadherin, which can bind CD103 (α E β 7 integrin) expressed by IELs (17), as well as tight junction proteins. For example, γδ IELs express several epithelial tight junction proteins, including occludin and zonula occludens-1 (ZO-1) (18), that may bind directly or indirectly t...

The intestinal epithelium restricts free passage of toxic and infectious molecules from the gut lumen while allowing selective paracellular absorption

A single epithelial layer serves as the interface between the organism and the contents of the gastrointestinal tract, underlining the importance of regulating cellular viability despite an onslaught of pathogens, toxins, waste by-products, and cytokines. A balance between cellular proliferation and apoptosis is necessary to maintain this critical barrier. Recent findings have begun to explain the mechanisms by which intestinal epithelial cells are able to survive in such an environment and how loss of normal regulatory processes may lead to inflammatory bowel disease (IBD) and predispose to inflammation-associated neoplasia. This review focuses on the regulation of physiological apoptosis in development and homeostasis and on pathological apoptosis in intestinal disease, inflammation, and neoplasia, identifying remaining questions and areas of needed investigation.

Background & Aims Intraepithelial lymphocytes that express the γδ T cell receptor (γδ IELs) limit pathogen translocation across the intestinal epithelium by unknown mechanisms. We investigated whether γδ IEL migration and interaction with epithelial cells promote mucosal barrier maintenance during enteric infection. Methods Salmonella typhimurium or Toxoplasma gondii were administered to γδ T cell-deficient (Tcrd KO), CD103-deficient (CD103 KO), or control TcrdEGFP C57BL/6 reporter mice. Intravital microscopy was used to visualize migration of GFP-tagged γδ T cells within the small intestinal mucosa of mice infected with DsRed-labeled S typhimurium. Mixed bone marrow chimeras were generated to assess the effects of γδ IEL migration on early pathogen invasion and chronic systemic infection. Results Morphometric analyses of intravital video microscopy data showed that γδ IELs rapidly localized to and remained near epithelial cells in direct contact with bacteria. Within 1 hr, greater numbers of T gondii or S typhimurium were present within mucosae of mice with migration-defective occludin KO γδ T cells, compared with controls. Pathogen invasion in Tcrd KO mice was quantitatively similar to that in mice with occludin-deficient γδ T cells, whereas invasion in CD103 KO mice, which have increased migration of γδ T cells into the lateral intercellular space, was reduced by 63%. Consistent with a role of γδ T cell migration in early host defense, systemic salmonellosis developed more rapidly and with greater severity in mice with occludin-deficient γδ IELs, relative to those with wild-type or CD103 KO γδ IELs. Conclusions In mice, intraepithelial migration to epithelial cells in contact with pathogens is essential to γδ IEL surveillance and immediate host defense. γδ IEL occludin is required for early surveillance that limits systemic disease.