Shigella flexneriInteractions with the Basolateral Membrane Domain of Polarized Model Intestinal Epithelium: Role of Lipopolysaccharide in Cell Invasion and in Activation of the Mitogen-Activated Protein Kinase ERK

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Abstract
An early step governingShigella flexneripathogenesis is the invasion of the colonic epithelium from the basolateral surface followed by disruption of the colonic epithelial barrier. Despite recent insight intoS. flexneri-host interactions, much remains to be determined regarding the nature of the initial contact betweenS. flexneriand the host epithelial basolateral membrane domain. Since the lipopolysaccharide (LPS) is located at the outermost part of the bacterial membrane, we considered that this component might be used byS. flexnerito attach to the basolateral surface of the intestinal epithelium and promote a proinflammatory response. Therefore, polarized human T84 intestinal epithelial cells were infected from the basolateral surface with either wild-typeS. flexnerior one of its isogenic LPS-defective strains with mutations in eitherrfc,rfaL, orgalU. We found that both adherence to and internalization into the basolateral surface of a polarized intestinal epithelium withS. flexneriwere highly dependent on the length of the LPS (i.e.,rfc>rfaL>galU). Furthermore, the addition of the anti-inflammatory LPS (RsDPLA) considerably decreased the invasion profile of wild-typeS. flexneriby nearly 50%. Since LPS is associated with host inflammation, we further examined whether this molecule was involved inShigella-induced inflammatory events. We found thatS. flexneriLPS plays an important role in mediating epithelial-derived signaling, which leads to directed migration of polymorphonuclear leukocytes across model intestinal epithelium. This signaling most likely involves the activation of the mitogen-activated protein kinase extracellular regulated kinase. Thus, our findings have important implications on the understanding of the mechanisms by whichS. flexnerican elicit mucosal inflammation.