Toll-Like Receptor 4 Expression and Cytokine Responses in the Human Urinary Tract Mucosa

Abstract

Mucosal pathogens trigger a local innate host response by activating epithelial cells. Bacterial adherence and Toll-like receptor 4 (TLR4) signaling have been implicated as key events in this process. This study addressed the molecular basis of the epithelial response to gram-negative infection in the human urinary tract. Mucosal biopsies were obtained from kidneys, ureters, and bladders of patients undergoing urinary tract surgery, and epithelial TLR4 and CD14 expression was examined by immunohistochemistry. TLR4 was detected in epithelial cells lining the entire urinary tract and in the renal tubular epithelium. CD14, in contrast, was completely absent from the epithelial tissue. The response of the epithelial cells to infection was studied by in vitro challenge of the biopsies with uropathogenic Escherichia coli bacteria. A rapid cytokine response was observed, with production of interleukin-1␤ (IL-1␤), IL-6, and IL-8 but not of IL-4 or gamma interferon. Adhering, P-or type 1-fimbriated E. coli activated IL-6 and IL-8 production more efficiently than the nonfimbriated control, as shown by cellular staining and analysis of secreted cytokines. The results demonstrate that human uroepithelial cells possess the molecular machinery needed to respond to uropathogenic E. coli. This includes recognition receptors for fimbriae and TLR4 for transmembrane signaling. We speculate that the lack of membrane-bound CD14 allows the epithelium to regulate its sensitivity to lipopolysaccharide and to discriminate between more-virulent and less-virulent strains.Mucosal pathogens use diverse and highly sophisticated mechanisms to gain access to the tissues at their preferred site of infection (6,8,11,35). Adherence is a crucial first step to establish tissue contact and to break the inertia of the mucosal barrier, but in addition, the molecular interactions between bacteria and host alert the host to the danger, and a host response is activated (1,14). In urinary tract cell lines, epithelial cell activation by fimbriated Escherichia coli requires primary recognition receptors for fimbrial adhesins and Toll-like receptor 4 (TLR4) for transmembrane signaling (12). Human urinary tract epithelial cells express both glycosphingolipid and mannosylated surface glycoprotein receptors, which recognize the P fimbrial adhesins (29) and the type1 fimbriae, respectively (30). TLR4 is also expressed in murine urinary tract epithelium, and the tlr4 genotype was found to regulate the in vivo response to experimental urinary tract infection (UTI) caused by P-or type 1-fimbriated E. coli (12,17,36,38).The extent to which TLR4 is expressed by the human urinary tract epithelium remains controversial, however. In addition, there are contradictory reports concerning the lipopolysaccharide (LPS) responsiveness of uroepithelial cells and their expression of CD14. It is well established that cells of myeloid origin express CD14 and MD2 and recruit TLR4 for transmembrane signaling when exposed to LPS (5), but uroepithelial cell lines respond poorly to sol...

Renal Collecting Duct Epithelial Cells React to Pyelonephritis-Associated Escherichia coli by Activating Distinct TLR4-Dependent and -Independent Inflammatory Pathways

“…Recent studies using the hemopoietic chimeric Lps n and Lps d mice demonstrated that bladder epithelial cells (19), as well as intrinsic renal epithelial cells (20), contribute together with bone marrow-derived cells to the initiation of antibacterial immunity. Murine and human renal tubule cells express some of the TLR expressed in hemopoietic cells and secrete chemokines upon LPS stimulation (21,22). In addition, TLR2 and TLR4 have been shown to be up-regulated in the distal nephron of inflamed, postischemic, reperfused kidneys (23).…”

Mechanism of pathogen‐specific TLR4 activation in the mucosa: Fimbriae, recognition receptors and adaptor protein selection

“…Epithelial cells must be protected from constant TLR activation by commensals and their PAMP, in order for mucosal integrity to be maintained. The epithelial cells thus lack co-receptors like CD14 [6][7][8][9][10][11], and in addition, commensals have been suggested to actively suppress epithelia responses through NF-jB [12,13]. Yet, mucosal pathogens evoke rapid TLR4-dependent responses at mucosal sites, suggesting that alternative ligands and receptors might be involved in mucosal TLR activation.…”

Integrated regulatory responses of fimB to N -acetylneuraminic (sialic) acid and GlcNAc in Escherichia coli K-12

“…Type 1 fimbriae are proinflammatory, stimulating release of IL-6, IL-8, and tumor necrosis factor ␣, and they act synergistically with other bacterial products such as lipopolysaccharide (8)(9)(10)(11)(12). The adhesin is sufficient for invasion of uroepithelial cells and may contribute to the etiology of the chronic inflammatory diseases, Crohn's disease, and interstitial cystitis (13)(14)(15)(16).…”