Dendritic cells express tight junction proteins and penetrate gut epithelial monolayers to sample bacteria

Penetration of the gut mucosa by pathogens expressing invasion genes is bel ieved to occur mainly through specialized epithelial cells, called M cells, that are located in Peyer's patches. However, Salmonella typhimurium that are deficient in invasion genes encoded by Salmonella pathogenicity island I (SPII) are still able to reach the spleen after oral administration. This suggests the existence of an alternative route for bacterial invasion, one that is independent of M cells. We report here a new mechanism for bacteri al uptake in the mucosa tissues that is mediated by dendritic cells (DCs). DCs open the tight junctions between epithelial cells, send dendrites outsi de the epithelium and directly sample bacteria. In addition, because DCs ex press tight-junction proteins such as occludin, claudin I and zonula occlud ens I,the integrity of the epithelial barrier is preserved.