Tasting for trouble
Our guts are swarming with bacteria and viruses. A paper in the April issue of Nature Immunology (Vol. 2, Issue 4, 01 Apr 2001) identifies a novel strategy that the immune system could use as an "early warning system" to keep us one step ahead in our ongoing struggle against intruders. Researchers from Italy and Switzerland, led by Paola Ricciardi-Castagnoli of the University of Milano-Bicocca, investigated how one type of immune cell, the dendritic cell, is able to keep the threat from gut pathogens at bay.
A single layer of epithelial cells lines our intestines; these cells are joined together by "tight junctions" through which nothing can pass. Although it is difficult for bacteria to penetrate through epithelial cells, another type of cell, the M cell, is interspersed in the epithelial layer and certain bacteria are able to cross over through these cells and into the body. Ricciardi-Castagnoli and colleagues found that the dendritic cells that reside below the epithelial cells are actually able to open the tight junctions of the epithelial cells and send processes into the intestinal lumen to directly sample bacteria, much like a submarine might scout the ocean's surface with a periscope. And although the dendritic cells wriggled through the old tight junctions, which one would imagine could be disastrous for the structural integrity of the intestinal wall, it turns out that the dendritic cells can generate new tight junctions, so that the epithelial barrier is not compromised.
Once dendritic cells pick up foreign matter, they alert the immune system that action is required to destroy the invaders. Although dendritic cells constantly sample their environment for invaders, this is the first demonstration that they can open tight junctions to directly access gut invaders. This discovery may explain how strains of Salmonella that lack the invasion genes required to cross M cells could still be found in the lymph system of mice that have swallowed the pathogen. This mechanism could be used by the immune system to induce an early immune response to pathogens.
A News & Views was written on this paper by James L. Madara of Emory University in Atlanta, GA, USA.
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Message posted by: Trevor M. D'Souza