Crohn's disease and the NOD2 gene: A role for paneth cells
The NOD2 gene, which is strongly associated with susceptibility to Crohn's disease (CD) of the terminal ileum, interacts with bacterial lipopolysaccharide (LPS), inducing cellular activation. However, the mechanisms by which NOD2 mutations cause terminal ileitis are unknown, and NOD2 is expressed most highly by peripheral blood monocytes, which are distributed ubiquitously and readily respond to LPS via cell-surface receptors. Paneth cells on the other hand, are most numerous in the terminal ileum, are critically important in enteric antibacterial defense, and respond to LPS through as yet undefined pathways. We therefore determined if these specialized intestinal epithelial cells also expressed the NOD2 gene. In situ hybridization, immunohistochemistry, and laser-capture microdissection were used to determine RNA and protein expression in tissue sections, and real-time reverse-transcription polymerase chain reaction (RT-PCR) was used to quantitate gene expression in intestinal epithelial cells and peripheral blood mononuclear cells. NOD2 was detected readily in monocytes, but not in mature macrophages in the lamina propria or within granulomas, and levels declined as monocytes differentiated into macrophages in vitro, so that Caco-2 cells expressed more NOD2 mRNA than macrophages. NOD2 mRNA was enriched in crypts compared with villi, and in situ, Paneth cells were the most prominent cells expressing NOD2 in normal and CD-affected intestinal tissue, where they also strongly expressed tumor necrosis factor alpha (TNFalpha) RNA. The NOD2 gene product is most abundant in Paneth cells in the terminal ileum, which could therefore play a critical and hitherto unrecognized role in the pathogenesis of NOD2-associated CD.