Antigen uptake by the gut

The gut is constantly challenged with large doses of soluble and particulate antigens. Although it is highly antigen-impermeable, the gut possesses specialized antigen transport mechanisms in the epithelium covering the Peyer's patches (PPs), the appendix and scattered lymphoid follicles. M cells are commonly interspersed between enterocytes; their numbers vary between species. M cells are physically attached to enterocytes via conventional junctional complexes. Although the intermediate filament composition and glycosylation state of M cells is slightly different from that of the surrounding enterocytes, the junctional complexes and the presence of epithelial-specific vimentin and cytokeratin expression in M cells provide evidence for an epithelial origin of M cells, in contrast to all other types of antigen-handling cells which are bone marrow derived. M cells differ from the rest of the epithelial cells in histochemical and ultrastructural respects. At the microscope level these cells can be recognized by their close association with IELs. M cells possess large basolateral pocked domains which harbour specialized IELs. In ultrathin sections, an M cell is seen as a rim of apical cytoplasm that bridges the space between the adjacent enterocytes (Figure 2). This M cell forms a kind of umbrella above a space in which IELs, and sometimes macrophages and dendritic cells, are present. Antigen is absorbed at the apical side of the M cell; absorption may be aided by the lack of a 'fuzzy coat' and the absence of mucous (goblet) cells in the PP epithelium. Following absorption, antigen is endo-cytosed and transported into vesicles to the lateral and basal cell membrane; by fusion the antigen is released into the basolateral space, where it comes into contact with lymphocytes and macrophages or dendritic cells. Antigen is transcytosed by M cells

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Figure 2 (A) A Peyers patch; (B) detail of an M cell within the Peyers patch epithelium. After uptake and transport by the M cell, antigen is processed by macrophages or by dendritic cells which transform into interdigitating cells (IDCs) and is then presented to lymphocytes in the T cell areas of Peyers patches and mesenteric lymph nodes (MLN). E, enterocyte; L, lymphocyte; m, macrophage/dendritic cell.

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