In vivo and in vitro, Shigella flexneri, an invasive pathogen of the human colon, cannot invade epithelial cells through their apical pole. To identify ways by which it may reach the cellular basolateral domain in order to invade, we have established an assay using the human colonic T-84 cell line grown on permeable filters. Human PMN were added to the basal pole of the cells, and invasive shigellae to their apical pole. Apical addition of bacteria induced strong transmigration of PMN, reaching a maximum after 1 h of incubation. Transmigration depended on a receptor-specific interaction since it was inhibited by an anti-CD18 monoclonal antibody that antagonizes binding of MAC 1 on its putative epithelial cell receptor. After 1 h of PMN transmigration, shigellae started to invade the monolayer in areas of intense PMN infiltration. Invasion was clearly dependant on PMN transmigration since it was also inhibited by addition of an anti-CD18 monoclonal antibody. This in vitro assay is consistent with in vivo observations showing early PMN efflux within colonic crypts in the course of shigellosis. PMN transmigration may therefore allow invasion in the colon by opening the paracellular pathway to invasive microorganisms. (J. Clin.
Shigella flexneri, a member of the family of enterobacteriaceae, causes bacillary dysentery by invading the human colonic mucosa and provoking a very intense inflammation. Recent in vitro data allow us to integrate different phenomena into a model of the infectious process during shigellosis. In vivo, bacteria appear to enter the submucosa via the M cells, specialized cells that cover the follicular structures of the intestinal mucosa. Once inside the submucosa, shigellae encounter resident tissue macrophages, which are infected, and apoptosis is rapidly induced. During programmed cell death the inflammatory cytokine interleukin-1 (IL-1) is released. Interleukin-1 triggers an inflammatory reaction characterized by extravasation of polymorphonuclear (PMN) cells. The inflammation is probably potentiated by the production of other cytokines by epithelial, endothelial, and PMN cells. Polymorphonuclear cells migrate through the epithelium into the lumen of the colon, destabilizing the integrity of the epithelial barrier. The damaged epithelium allows massive entry of bacteria into the submucosa. Further colonization of the epithelium aggravates inflammation, which in turn causes extensive tissue destruction. Both the in vitro and in vivo results that support this model are discussed.
Mesenchymal cells (fibroblasts, smooth muscle cells) and endothelial cells were shown to interact with elastin fibers. The strong adhesion of elastin fibers to these cells is mediated by a cell membrane complex with a major glycoprotein component of 120 kDa designated as elastonectin. This interaction was studied by transmission electron microscopy (TEM) and immunocytochemical techniques using antibodies raised against the elastin adhesive proteins. When fibroblasts and smooth muscle cells were cultured in presence of elastin fibers, TEM showed an adhesion mechanism that takes place over several sites along the plasma membrane of these cells. Endothelial cells showed a very close association with elastin, emitting "pseudopodia" that embody the fibers. TEM, indirect immunofluorescence, immunoperoxidase, and confocal microscopy showed the presence and localization of cell membrane components synthesized in large quantities when cells were incubated in presence of elastin. Cells without elastin fibers barely revealed the adhesive membrane complex. These results confirm and extend previous findings concerning the presence of an inducible cell membrane complex that mediates the adhesion of elastin fibers to these cell types.
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