M cells (or microfold cells) are cells found in the follicle-associated epithelium of the Peyer's patch as well as in bronchus-associated lymphoid tissue (BALT). They transport organisms and particles from the gut lumen to immune cells across the epithelial barrier, and thus are important in stimulating mucosal immunity.
Unlike their neighbouring cells, they have the unique ability to take up antigen from the lumen of the small intestine via endocytosis or phagocytosis, and then deliver it via transcytosis to dendritic cells (an antigen presenting cell) and lymphocytes (namely T cells) located in a unique pocket-like structure on their basolateral side.
Structure and function
M cells differ from normal enterocytes in that they lack microvilli on their apical surface, but instead possess broader microfolds that give the cell its name. These cells are far less abundant than enterocytes. M cells do not secrete mucus or digestive enzymes, and have a thinner glycocalyx, which allows them to have easy access to the intestinal lumen for endocytosis of antigens. M cells main function is the selective endocytosis of antigens, and transporting them to intraepithelial macrophages and lymphocytes, which then migrate to lymph nodes where an immune response can be initiated.
M cells are exploited by several pathogens, including Shigella flexneri, Salmonella typhimurium, and Yersinia pseudotuberculosis, as well as infectious prions in Bovine spongiform encephalitis (Mad-cow disease), as a way to penetrate the intestinal epithelium. Exploitation as a virulence factor depends upon the pathogen's ability to bind to M cells and thus guarantee penetration in that manner, as M cells sample intestinal contents. EPEC (see Pathogenic Escherichia coli) containing plasmids with genes for EAF (Escherichia coli Adherence Factor) will adhere to M cells.
They are also exploited by viruses such as Polio and Reovirus for dissemination.
Factors promoting the differentiation of M cells have yet to be elucidated, but they are thought to develop in response to signals from immune cells found in the developing Peyer's patch.
- Laurent Ouzilou1, Elise Caliot2, Isabelle Pelletier1, Marie-Christine Prévost3, Eric Pringault2 and Florence Colbère-Garapin1. Journal of General Virology (2002), 83, 2177-2182.
- Grigorios Fotopoulos*, Alexandre Hararidagger , Pierre MichettiDagger , Didier Trono§, Giuseppe Pantaleodagger, and Jean-Pierre Kraehenbuhl. July 1, 2002, 10.1073/pnas.142586899
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