Probiotic formulations are widely available and have a variety of proposed beneficial effects, including promotion of gut health. The mechanisms of action of probiotic bacteria in the intestine are still unclear but are generally attributed to an antiinflammatory effect. Here, we demonstrate that the multiple probiotic formulation VSL#3 prevents the onset of intestinal inflammation by local stimulation of epithelial innate immune responses (i.e., increased production of epithelial-derived TNF-α and restoration of epithelial barrier function in vivo). We also demonstrate that probiotic bacteria stimulate epithelial production of TNF-α and activate NF-κB in vitro. Our results support the hypothesis that probiotics promote gut health through stimulation, rather than suppression, of the innate immune system. Furthermore, our findings provide the perspective that defects in innate immunity may play a critical role in the pathogenesis and progression of intestinal disorders, such as inflammatory bowel disease.

The SAMP1/YitFc mouse strain represents a model of Crohn's disease (CD)-like ileitis that is ideal for investigating the pathogenesis of chronic intestinal inflammation. Differently from the vast majority of animal models of colitis, the ileal-specific phenotype characteristic of SAMP1/ YitFc mice occurs spontaneously, without genetic, chemical or immunological manipulation. In addition, SAMP1/YitFc mice possess remarkable similarities to the human condition in regard to disease location, histologic features, incidence of extra-intestinal manifestations, and response to conventional therapies. SAMP1/YitFc mice also display a well-defined time course of a predisease state, and phases of acute and chronic ileitis. As such, the SAMP1/YitFc model is particularly suitable for elucidating pathways that precede the clinical phenotype that may lead to preventive, and therefore more efficacious, intervention with the natural course of disease, or alternatively, for the development of therapeutic strategies directed against chronic, established ileitis. In the following review, we summarize important contributions made by our group and others that uncover potential mechanisms in the pathogenesis of CD using this unique murine model of chronic intestinal inflammation.

Regulation of messenger RNA stability by AU-rich elements is an important means of regulating genes induced by growth factors and cytokines. Nup475 (also known as tristetraprolin, or TIS11) is the prototype for a family of zinc-binding Cys 3 His motif proteins required for proper regulation of tumor necrosis factor mRNA stability in macrophages. We developed an Escherichia coli expression system to produce soluble Nup475 protein in quantity to study its RNA binding properties. Nup475 protein bound a tumor necrosis factor AU-rich element over a broad range of pH and salt concentrations by RNA gel shift. This binding was inhibited by excess zinc metal, providing a potential mechanism for previous reports of zinc stabilization of AU-rich element (ARE) containing messenger RNAs. Immobilized Nup475 protein was used to select its optimal binding site by RNA SELEX and revealed a strong preference for the extended sequence UUAUUUAUU, rather than a simple AUUUA motif. These findings were confirmed by sitedirected mutagenesis of the tumor necrosis factor ARE and RNA gel shifts on c-fos, interferon-␥, and interferon-␤ ARE fragments. A weaker binding activity toward adenine-rich sites, such as a poly(A) tail RNA fragment, can partially disrupt the Nup475-tumor necrosis factor AU-rich element complex.

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