Characterization of M cell development during indomethacin-induced ileitisin rats

Background: M cells play an important role in the intestinal immune system as they have a high capacity for transcytosis of a wide range of microorgan isms and macromolecules. However, little is known about the role of M cells during intestinal inflammation. Aim: We studied M cell development during indomethacin-induced intestinal i nflammation in rats. Methods: Ileitis in rats was induced by two subcutaneous injections with in domethacin (7.5 mg/kg) given 24 h apart. Rats were sacrificed after 14 days and tissue was analysed by fluorescence microscopy and electron microscopy . M cells could be visualized by using the FITC-labelled mAb anti-cytokerat in (CK)-8 (clone 4.1.18), which was recently identified as specific M cell marker in rats. The number of cytokeratin-8 positive M cells was related to the surface of the follicle associated epithelium. For morphological studi es, we used both transmission electron microscopy (T.E.M.) and scanning ele ctron microscopy (S.E.M.). Results: In non-inflamed ileum M cells were scarce. Only 4% of the follicle associated epithelium were M cells, whereas an increase of M cells up to 1 1% was found in inflamed follicle associated epithelium (P < 0.001). The ra te of M cell induction depended on the macroscopic degree of inflammation. T.E.M./S.E.M. studies showed that in inflamed tissue most M cells underwent apoptosis with typical morphological signs. In contrast to apoptotic M cel ls, the neighbouring enterocytes usually appeared intact. The number of mon onuclear cells below the follicle associated epithelium was significantly i ncreased. S.E.M. studies revealed that during induced ileitis mononuclear c ells migrated from the lamina propria into the gut lumen by passing through apoptotic M cells. Conclusions: During indomethacin-induced ileitis in rats the increase in M cell number in association with apoptosis of M cells may alter the intestin al barrier function. These observations may play a pivotal role in the path ogenesis of chronic intestinal inflammation, e.g. in inflammatory bowel dis ease.