Salmonella Infection Induces a Hypersecretory Phenotype in Human Intestinal Xenografts by Inducing Cyclooxygenase 2

The hallmark of a range of intestinal infectious diseases is diarrhea. With the exception of intestinal pathogens that elaborate enterotoxins, the mechanisms underlying intestinal dysfunction during infections with many bacteria remain largely unestablished. The intestinal epithelium is the initial site of contact between enteric microorganisms and the gastrointestinal mucosa and is a continuous layer of cells lining the entire gastrointestinal tract. It acts as a physical barrier, preventing the entry of harmful substances from the intestine into the body, and transports water to and from the intestinal contents. Water transport is a passive process, which is driven by the active transport of ions and nutrients. Normally, there is a net absorption of ions and water but in conditions of disease, such as intestinal bacterial infection, the epithelium can rapidly convert from its absorptive to a net secretory phenotype. Salmonellae are gram-negative bacteria that can cause gastroenteritis and enteric fever and represent a major public health problem worldwide. All Salmonella serotypes share the ability to invade their host by inducing their own uptake into nonphagocytic cells of the intestinal epithelium. They facilitate this uptake through proteins secreted by a type III protein secretion system encoded within the Salmonella pathogenicity island 1 that also encodes the effector proteins required for invasion (13). This system is abundant in several bacteria and, in addition to being responsible for invasion, functions to secrete virulence proteins into host cells. Thus, through secreted virulence proteins, through invasion, or both, Salmonella can induce changes in the epithelium, altering the function of the intestinal lining and presumably converting it to a hypersecretory state. However, the underlying mechanism(s) by which Salmonella induces diarrhea are not fully understood. The understanding of diarrheal disease requires the application of model systems. Animal tissues are commonly used but have a number of shortcomings due to significant species differences between experimental animals and human hosts. Moreover, studies with isolated epithelial cell lines lack relevant cell-cell interactions. Thus, new models for studying intestinal bacterial infections are being sought. In this regard, we have previously described a human-derived tissue model for the study of ion transport mechanisms (4). This model employs fetal intestinal tissues grown as xenografts in severe combined immunodeficient (SCID) mice (22). The main advantage of intestinal xenografts is that they provide a model consistent with intact human intestinal tissue, which allows for prolonged infection with bacteria and enables studies of the interactions of different cell types, in both human small and large intestine, during bacterial infection. Infection studies with epithelial cell lines have shown that Salmonella induces host cell responses, including chemokine and cytokine secretion (8, 9), and upregulation of expression of cyclooxygenase 2 (Cox-2) (8, 21). Coculture studies also show transepithelial migration of polymorphonuclear leukocytes early after Salmonella-epithelial cell contact (17) and that epithelial invasion by Salmonella is not necessary for polymorphonuclear leukocyte movement (11). However, changes in the secretory phenotype of intestinal epithelial cell lines show a considerable latency after Salmonella infection and are difficult to dissociate from frankly injurious effects of the infectious process (21). These various observations suggest that interactions between intestinal epithelial cells and Salmonella play a key role in orchestrating the inflammatory response (9, 17, 18) and underscore the limitations inherent in cell line models to understand the full picture of Salmonella infection in human tissue. It is therefore relevant to the elucidation of Salmonella pathogenesis to utilize the intestinal xenograft model to extend findings obtained with cell lines to a more physiological setting, in which cell-cell interactions may amplify responses to the pathogen. Thus, we sought to examine here whether Salmonella infection of xenografts had any consequences in terms of transport function and the characteristics and mechanisms of any responses seen. (This work was presented in part at the 2001 meeting of the American Gastroenterological Association and has been published in abstract form [Gastroenterology 120:A3812, 2001].)