Rapid appearance of M cells after microbial challenge is restricted at theperiphery of the follicle-associated epithelium of Peyer's patch

M cells within the follicle-associated epithelium (FAE) of the gut play a c entral role in the initiation of mucosal immune responses by transporting a ntigens to the intestinal lymphoid tissue. We have previously demonstrated that the instillation into the gut of a nonenteric microorganism, Streptoco ccus pneumoniae R36a, is an excellent experimental model to investigate the highly dynamic nature of the FAE in response to microbial challenge. In th e present study, S. pneumoniae was introduced into rabbit ileal loops, each one containing a Peyer's patch (PP), and the number of M cells was assesse d by morphological and functional characteristics in different areas of the FAE after a short time (1-3 hours). We report that a marked increase in th e number of M cells was detected in the periphery, but not in the apical ar ea, of the FAE as early as 1 hour after exposure to S. pneumoniae. Furtherm ore, a variant of this experiment enabled us to establish that the increase d numbers of M cells led to an improved capability of the FAE to transport latex fluorescent microspheres (0.5 mu m), highly specific to rabbit M cell s, from the gut lumen to the intestinal lymphatic system. In these animals the cisterna chyli was cannulated, and the microparticles were introduced i nto the intestinal loops after stimulation with pneumococci. The microparti cles reaching the lymph were then counted by flow cytometer. We interpreted these results as showing that only enterocytes located within the peripher y of the FAE are converted to fully operational M cells by certain microbia l interaction and the ability of enterocytes to undergo this conversion may depend on their stage of differentiation.