54 The cellular pathways of infection utilized by pathogenic enteric bacteria have important implications for their clinical manifestations. Yersinia reaches Peyer’s patches via M cells and uses plasmid-encoded factors to resist phagocytic cells. Shigella also translocates via M cells and incapacitates phagocytes, but subsequently re-enters the epithelium basolaterally to elicit an acute inflammatory response. Salmonella has recently been shown to both colonize Peyer’s patches via M cells and independently disseminate to extraintestinal sites via CD18-expressing phagocytes. M cell- mediated entry can lead to gastroenteritis and mucosal antibody production, while systemic dissemination can result in septicemia and elicitation of systemic immune responses. Addresses Departments of Medicine, Pathology and Microbiology, University of Colorado Health Sciences Center, 4200 E. 9 th Avenue, B168, Denver, CO 80262, USA *e-mail: ferric.fang@uchsc.edu Current Opinion in Microbiology 2000, 3:54–59 1369-5274/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved. Abbreviations DC dendritic cells FAE follicle-associated epithelium GFP green fluorescent protein IL interleukin MLN mesenteric lymph node SPI1 Salmonella pathogenicity island 1 TNFα tumor necrosis factor α Introduction “That inv mutants were able to penetrate into deep tissues without rapid localization into regional lymph nodes indicates that there exists a second invasin-independent pathway responsible for translocation across the intestinal mucosa.’’ RR Isberg [1] A single epithelial cell layer separates the host from the lumen of the gastrointestinal tract. In addition to its primary digestive and absorptive functions, the gut-associ- ated epithelium provides an effective barrier that deters commensal, opportunistic and pathogenic microorganisms from establishing infection. The densely packed glycoca- lyx, mucus coat and apical acidic microenvironment of the intestinal epithelium, together with cell turnover, peristal- sis, gastric acidity, lactoferrin, lysozyme, and other antimicrobial peptides, effectively limit microbial access to underlying intestinal mucosa [2]. Furthermore, lymphoid tissue organized in discrete collections under follicle-asso- ciated epithelium (FAE) is dispersed throughout the mucosal lamina propria and intercalated between entero- cytes to afford specific humoral and cellular protection against mucosal pathogens. Despite this plethora of antimicrobial mechanisms, a number of pathogens have devised strategies to attach, invade and translocate across the gut epithelium. Herein we present a brief overview of the cellular routes of invasion exploited by enteropatho- genic Yersinia, Shigella and Salmonella species. M cell route of invasion Terminally differentiated M cells specialize in sampling and transporting luminal macromolecules, particulate mat- ter and microbes across FAE to an underlying T lymphocyte-, B lymphocyte-, macrophage- and dendritic cell-rich dome area [3–5]. Reduced brush border and gly- cocalyx in the apical membranes of M cells facilitate close contact with luminal antigens. In addition, unique patterns of glycosylation of the M cell surface are thought to con- tribute to their antigenic selectivity and microbial accessibility [6,7]. For example, human, rabbit and mouse apical M cell membranes are rich in sialyl Lewis A antigen, mucin-related antigens, and β1 integrins, respectively, in comparison to neighboring epithelial cells [8 • ,9 • ,10 •• ]. M cells occur exclusively in the context of associated lym- phoid tissue. It has been recently discovered that this anatomical relationship is the consequence of lymphocyte- induced differentiation of enterocytes into FAE and M cells [11,12]. M cells develop on the follicular side of dome-associated crypts [13]. Studies with immunodefi- cient mice indicate that B cell factors induce M cell differentiation. Pringault and collaborators [11] have devised a method for deriving M cells in vitro based on the co-culture of murine Peyer’s patch lymphocytes with the human enterocyte cell line Caco-2. M cell in vitro cultures should greatly facilitate the identification of novel strate- gies to prevent microbial invasion and elicit immune responses to pathogens acquired via the gastrointestinal tract. This technique also promises to provide a useful model system for analysis of the complex cytoskeletal rearrangements and signaling events that are likely to be induced by enteroinvasive bacteria upon M cell entry. Since 1922 when Kenzaburo Kumagai discovered that mycobacteria could be ingested by the dome epithelium of intestinal lymphoid tissue (reviewed in [4]), at least 12 bac- terial species and a variety of viruses and protozoa have been found to be taken up by M cells [5], including the enteropathogens Yersinia, Shigella, and Salmonella. Yersinia Yersinia enterocolitica and Y. pseudotuberculosis cause a variety of local and systemic clinical syndromes including enteri- tis, mesenteric lymphadenitis, septicemia and reactive arthritis. These infections each commence with M cell invasion and colonization of ileal Peyer’s patches [14] Cellular routes of invasion by enteropathogens Andrés Vazquez-Torres and Ferric C Fang*