M cells as ports of entry for enteroinvasive pathogens: Mechanisms of interaction, consequences for the disease process

M cells are key sites of antigen sampling for the mucosal-associated lympho id system (MALT) and consequently are essential components of the structure s serving as inductive sites for mucosal immunity. In addition, they have r ecently been recognized as major sites of adherence and major ports of entr y for enteric pathogens. In the case of enteroinvasive pathogens, such as S almonella, Yersinia and Shigella, few clinical evidences, but lots of exper imental data indicate that, at bast at the early stage of infection, M cell s of the follicular associated epithelium transport the pathogens. This has significantly altered our view on the pathogenesis of enteroinvasive infec tions. Crossing the epithelial barrier seems an achievable task for these b acteria which express adherence and invasion mechanisms which have often be en well characterized in epithelial cell lines. These systems seem to be al so used for entering and crossing M cells, although reproducible in vitro a ssays for M cell infection are now required. Having crossed the epithelial lining, the bacteria face phagocytic cells, particularly the macrophages th at are present in the follicle dome. Depending on the capacity to survive i n the presence of macrophages, and how this survival is achieved by a given invasive species, the outcome of infection can be dramatically affected. I n consequence, M cells can be considered as pathogen translocators toward i mmunocompetent areas of the gut, thus opening the possibility to harness th is property in order to design neu, mucosal vaccines or vaccine vectors.