An immunohistochemical study and review of potential markers of human intestinal M cells.

M cells are found in intestinal follicle associated epithelium. Studies into the physiological and pathological roles of human M cells have been hampered by the lack of well-substantiated, specific markers for these cells. A critical literature review suggests the following molecules may potentially serve as such markers: CK7, FcaR (CD89), S100, CD1a, CD21, CD23, sialyl Lewis A, and cathepsin E. Normal ileum, appendix and colorectum were studied using paraffin-embedded, formalin-fixed tissue and immunohistochemistry for these 8 markers. Cathepsin E immunohistochemistry was also performed on cases of colorectal adenocarcinoma, colorectal adenoma, colorectal hyperplastic/metaplastic polyp, lymphocytic colitis, collagenous colitis, pseudomembranous colitis and active ulcerative colitis. Of the 8 markers tested, only cathepsin E appeared to be specific to follicle associated epithelium (expressed by cells with and without M cell morphology) and follicular crypt epithelium; this specificity was limited to the colorectum. Focal epithelial expression of cathepsin E was seen in adenocarcinoma, adenoma, hyperplastic/metaplastic polyp, ulcerative colitis and pseudomembranous colitis. In conclusion, cathepsin E is a specific marker of normal colorectal follicle associated epithelium and follicular crypt epithelium though is not specific to M cells within these compartments. None of the other 7 markers studied is exclusively expressed by human M cells.