Abstract
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-γ 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-γ. The critical role of IFN-γ 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-γ into mutant mice lacking IEL showed that IFN-γ 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-γ and TNF release. This insures the rapid epithelial renewal of the villi, which in turn helps maintain the functional integrity of the barrier.
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