To clarify the immunological function of 'M' (microfold or membranous) cells in the large intestine, we examined the expression of intercellular adhesion molecule-1 (ICAM-1) and HLA-class II antigens immunohistochemically in M cells and follicle-associated epithelia (FAE) covering isolated lymphoid follicles of the human colon in comparison with their expression in Peyer's patches of the small intestine. In Peyer's patches of the small intestine, ICAM-1 was not expressed on the epithelial cells covering the lymphoid follicles, but their cell surfaces were stained positively for HLA-DR. In contrast, colonic M cells expressed ICAM-1 on their cell surfaces but were negative for HLA class II antigens. By immunoelectron microscopy, ICAM-1 was seen to be distributed on the surface of microfolds, on the membranes of apical vesicles and on part of the basolateral plasma membranes of M cells, but was not expressed on adjacent FAE. These findings imply that the M cells in the colon and in Peyer's patches have different immunological roles. In addition, identification of ICAM-1 expression on the colonic M cells should help elucidate the pathogenesis of some inflammatory colonic diseases which appear to start in the lymphoid follicles of the colonic mucosa.</p
