Small bowel enteropathy: role of intraepithelial lymphocytes and of cytokines (IL-12, IFN-gamma, TNF) in the induction of epithelial cell death and renewal.

The small bowel mucosa contains within its villus epithelium a large number of intraepithelial lymphocytes (IEL) which upon activation are cytotoxic and release large quantities of IFN-gamma and TNF; these activities are increased by in vitro exposure to IL-12. Mice injected with IL-12 develop severe damage of the villus epithelial cells, in form of apoptosis, necrosis and a third distinct form of cell death, characterized ultrastructurally by progressive cell shrinkage. These lesions are accompanied by a compensatory acceleration of the epithelial renewal, a hallmark of epithelial injury. Use of a variety of mutant mice showed that these lesions require the presence of IEL (all populations being involved, thymus-dependent as well as natural killer-T cell IEL) and the release of IFN-gamma. The critical role of IFN-gamma may result in part from its capacity to induce on epithelial cells the expression of target molecules involved in the different cytotoxic pathways used by IEL. However, injection of IFN-gamma into mutant mice lacking IEL showed that IFN-gamma can directly induce villus epithelial damage as well. On the other hand, injection of TNF induces fulminant apoptosis of villus epithelial cells, starting at the top of the villi; however TNF is not required for IL-12-induced enteropathy, which is unmodified in mutant mice lacking TNF. We propose that, when activated by their cognate ligands and/or IL-12 produced by cells in the lamina propria, IEL eliminate infected and senescent epithelial cells through a combination of cytotoxicity and of IFN-gamma and TNF release. This insures the rapid epithelial renewal of the villi, which in turn helps maintain the functional integrity of the barrier.