Michelle Benjamin

Role of mast cells in chronic stress induced colonic epithelial barrier dysfunction in the rat

BACKGROUND AND AIMS Stress may be an important factor in exacerbating inflammatory bowel disease but the underlying mechanism is unclear. Defective epithelial barrier function may allow uptake of luminal antigens that stimulate an immune/inflammatory response. Here, we examined the effect of chronic stress on colonic permeability and the participation of mast cells in this response. METHODS Mast cell deficient Ws/Ws rats and +/+ littermate controls were submitted to water avoidance stress or sham stress (one hour/day) for five days. Colonic epithelial permeability to a model macromolecular antigen, horseradish peroxidase, was measured in Ussing chambers. Epithelial and mast cell morphology was studied by light and electron microscopy. RESULTS Chronic stress significantly increased macromolecular flux and caused epithelial mitochondrial swelling in +/+ rats, but not in Ws/Ws rats, compared with non-stressed controls. Stress increased the number of mucosal mast cells and the proportion of cells showing signs of activation in +/+ rats. No mast cells or ultrastructural abnormalities of the epithelium were present in Ws/Ws rats. Increased permeability in +/+ rats persisted for 72 hours after stress cessation. CONCLUSIONS Chronic stress causes an epithelial barrier defect and epithelial mitochondrial damage, in parallel with mucosal mast cell hyperplasia and activation. The study provides further support for an important role for mast cells in stress induced colonic mucosal pathophysiology.

CD4+ T cells mediate superantigen-induced abnormalities in murine jejunal ion transport

The immunomodulatory properties of bacterial superantigens (SAgs) have been defined, yet comparatively little is known of how SAgs may affect enteric physiology. Staphylococcus aureus enterotoxin B (SEB) was used to examine the ability of SAgs to alter epithelial ion transport. BALB/c mice, severe combined immunodeficient (SCID, lack T cells) mice, or SCID mice reconstituted with lymphocytes or CD4+ T cells received SEB intraperitoneally, and jejunal segments were examined in Ussing chambers; controls received saline only. Baseline short-circuit current ( I sc, indicates net ion transport) and I sc responses evoked by electrical nerve stimulation, histamine, carbachol, or forskolin were recorded. Serum levels of interleukin-2 (IL-2) and interferon-γ (IFN-γ) were measured. SEB-treated BALB/c mice showed elevated serum IL-2 and IFN-γ levels, and jejunal segments displayed a time- and dose-dependent increase in baseline I sc compared with controls. Conversely, evoked ion secretion was selectively reduced in jejunum from SEB-treated mice. Elevated cytokine levels and changes in jejunal I sc were not observed in SEB-treated SCID mice. In contrast, SCID mice reconstituted with T cells were responsive to SEB challenge as shown by increased cytokine production and altered jejunal I sc responses that were similar to those observed in jejunum from SEB-treated BALB/c mice. We conclude that exposure to a model bacterial SAg causes distinct changes in epithelial physiology and that these events can be mediated by CD4+ T cells.

The role of mast cells in intestinal immunophysiology.

Immunophysiology of the gut is the study of the regulation/modulation of its function by immune cells and factors. Fluid secretion is a key function of the intestine which aids digestion by maintaining the lumenal contents in a liquid state, promoting mixing of nutrients with digestive enzymes which are then presented to the absorptive epithelial cells. Fluid secretion is also a host defence mechanism of mucosa that serves to wash away noxious material from the epithelial surface. In the gut, active transport of negatively charged Cl- ions into the lumen by epithelial cells has been clearly shown to be the driving force for fluid secretion, while positively charged ions such as Na+ follow in response to the electrical gradient. Cl- channels open in response to increased concentrations of intracellular second messengers such as cAMP and Ca2+, resulting in ion and fluid movement across the gut wall.1