Towards targeting strategies for oral immunization--identification of marker antigens in rat M cells

Specialized M cells of the follicle-associated epithelium (FAE) of Peyer's patches in the gastrointestinal and respiratory tracts play a crucial role in the immune surveillance of mucosal surfaces and are essential for the induction of mucosal immune responses. These cells transport luminal antigens to the underlying germinal centers and thereby initiate an immune response or induce tolerance. Mucosal immune responses could be significantly enhanced if oral antigens could be directly targeted to the apical surface of M cells. However, thus far M cells have not been isolated and suitable surface markers have not been described. For the characterization and identification of potential molecular markers of M cells the follicle associated epithelium of Peyer's patches from the small intestine of rats was analyzed by indirect immunofluorescence and immunogold electronmicroscopy. The expression of intermediate filament proteins in the FAE was found to be heterogeneous. Immunoreactivity of a specific monoclonal cytokeratin 8 antibody (clone 4.1.18) was selectively found in cells which were additionally characterized by the lack of staining for alkaline phosphatase in their apical membranes. In the FAE this property is an accepted criterium for the presence of M cells. The antigen recognized by the clone 4.1.18 is expressed in substantially higher amounts in M cells of Peyer's patches as compared to neighbouring epithelial cells or "normal" enterocytes and thus can be employed as an intracellular molecular marker for M cells of Peyer's patches in rats. It is expected, that this marker will be very helpful for the isolation of M cells.