Structural and functional dynamics of M-cell

Microfold cells (M-cells) of the follicle-associated epithelium (FAE) of the mucosa-associated lymphoid tissue (MALT) in gut play an important role in the genesis of both mucosal and systemic immune responses by delivering antigenic substrate to the underlying lymphoid tissue where immune responses start. A major branch of the immune system operates in mucosal tissues, providing these surfaces with protective secretory antibodies. The M-cells are restricted to the dome epithelium and transcytose not only antigens but also entire microorganisms from the lumen to the gut associated lymphoid tissue. Microfold cells are currently identified by the histochemical markers such as lectins and antibodies against the cytokeratins 8, 18 and 19 under the light microscopy. They are unequivocally epithelial cells and are interspersed among the enterocytes of the dome epithelium. Two different theories on M cell genesis in the FAE have been formulated. First, it was postulated that M-cells may originate in the crypts as a distinct cell lineage from stem cells via an independent differentiation programme; alternatively, it was thought that they may be formed by the conversion of FAE enterocytes upon interaction with the local lymphoid micro-environment. The brush border of M cell is poorly organized with short irregular microvilli, and the thick glycocalyx associated with absorptive cells is absent. These adaptations allow easy access of luminal material to the apical domain of M-cells, where it is internalized and then transported to the underlying lymphoid tissue. Unlike other intestinal epithelial cells, a characteristic of M-cells is that, they possess deeply invaginated basolateral surfaces that form intraepithelial pockets harbouring a wide variety of lymphocyte subsets that migrate to this unique compartment from lymphoid tissue. And this M cell pockets facilitate the contact between the incoming antigens and the specialized immune system in an environment separated from regulatory elements of the mucosal immune systems. It was also reported that M-cells are provided with enzymatic components, including cathepsin E typical of antigen-presenting cells. Although the presence of major histocompatibility complex (MHC) class II molecules remains controversial, it, was reported by some authors that M-cells expressed Ia molecules on the basolateral plasma membrane and organelles, such as prelysosome, lysosome and endosome and the expression of which was enhanced by pretreatment with interferon (IFN-ã). The notion that M-cells are not only simple conduits for antigen transport and may have a more active role in the early phase of mucosal immune responses is also suggested by their ability to produce immunoregulatory cytokines, such as interleukin IL-1. Although some vesicles in the M cell apical cytoplasm contain the endosomal protease cathepsin E, the late endosome/lysosome membrane marker IgP 120 and generate an acidic internal milieu, antigens and microorganisms are generally delivered intact and alive across M-cells.