The 'innate' host response protects and damages the infected urinary tract

Symptoms of infection and tissue pathology are caused by the host response; not by the microbe per se. The same response is also critical for the defence and is needed to clear infection. It is therefore essential to understand how the host response is activated and to identify the critical effector mechanisms of the defence. We have studied these issues in the urinary tract infection (UTI) model. The symptoms of UTI and the host defence both rely on the so-called 'innate' immune system, making this one of the best characterized human disease models of 'innate immunity. We discuss the critical molecular events that determine whether the host response will be activated by P-fimbriated uropathogenic Escherichia coli as well as factors determining whether the patient develops acute pyelonephritis or asymptomatic bacteriuria. We will describe the glycoconjugate receptors used by the P-fimbriated bacteria adhering to host tissues, the recruitment of TLR4 co-receptors and the signalling pathways that allow progression to symptomatic disease, and discuss how these mechanisms are altered in asymptomatic carriers, presenting the possible genetic basis for unresponsiveness. We have shown that neutrophils are the critical effectors of the host defence and that neutrophil dysfunctions lead to acute pyelonephritis and renal scarring. Here we discuss the mechanisms of neutrophil-mediated, chemokine receptor (CXCR1)-dependent clearance, and the defect in interleukin-8 receptor homolog knock-out (IL-8Rh KO) mice and describe the data linking low CXCR1 expression to recurrent pyelonephritis in man, as well as the information on the genetic basis for low CXCR1 expression in affected patients. Finally, the mechanisms of renal scarring in IL8Rh KO mice will be discussed in relation to human disease. Our studies hold the promise to provide a molecular and genetic explanation for disease susceptibility in some patients with UTI and to offer more precise tools for the diagnosis and therapy of these infections.

Neutrophil recruitment, chemokine receptors, and resistance to mucosal infection

Neutrophil migration to infected mucosal sites involves a series of complex interactions with molecules in the lamina propria and at the epithelial barrier. Much attention has focussed on the vascular compartment and endothelial cells, but less is known about the molecular determinants of neutrophil behavior in the periphery. We have studied urinary tract infections (UTIs) to determine the events that initiate neutrophil recruitment and interactions of the recruited neutrophils with the mucosal barrier. Bacteria activate a chemokine response in uroepithelial cells, and the chemokine repertoire depends on the bacterial virulence factors and on the specific signaling pathways that they activate. In addition, epithelial chemokine receptor expression is enhanced. Interleukin (IL)-8 and CXCR1 direct neutrophil migration across the epithelial barrier into the lumen. Indeed, mIL-8Rh knockout mice showed impaired transepithelial neutrophil migration, with tissue accumulation of neutrophils, and these mice developed renal scarring. They had a defective antibacterial defense and developed acute pyelonephritis with bacteremia. Low CXCR1 expression was also detected in children with acute pyelonephritis. These results demonstrate that chemokines and chemokine receptors are essential to orchestrate a functional antimicrobial defense of the urinary tract mucosa. Mutational inactivation of the IL-8R caused both acute disease and chronic tissue damage.

Escherichia coli P fimbriae utilize the Toll-like receptor 4 pathway for cell activation

Fimbriae mediate bacterial attachment to host cells and provide a mechanism for tissue attack. They activate a host response by delivery of microbial products such as lipopolysaccharide (LPS) or through direct fimbriae-dependent signalling mechanisms. By coupling to glycosphingolipid (GSL) receptors, P fimbriae trigger cytokine responses in CD14 negative host cells. Here we show that P fimbriae utilize the Toll-like receptor 4 (TLR4)-dependent pathway to trigger mucosal inflammation. Escherichia coli strains expressing P fimbriae as their only virulence factor stimulated chemokine and neutrophil responses in the urinary tract of TLR4 proficient mice, but TLR4 defective mice failed to respond to infection. Mucosal cells were CD14 negative but expressed several TLR species including TLR4, and TLR4 protein was detected. Infection with P fimbriated bacteria stimulated an increase in TLR4 mRNA levels. The activation signal did not involve the LPS-CD14 pathway and was independent of lipid A myristoylation, as shown by mutational inactivation of the msbB gene. Co-staining experiments revealed that TLR4 and the GSL receptors for P fimbriae co-localized in the cell membrane. The results demonstrate that P fimbriae activate epithelial cells by means of a TLR4-dependent signalling pathway, and suggest that GSL receptors for P fimbriae can recruit TLR4 as co-receptors.

Type 1 fimbriae deliver an LPS- and TLR4-dependent activation signal to CD14-negative cells

Fimbriae target bacteria to different mucosal surfaces and enhance the inflammatory response at these sites. Inflammation may be triggered by the fimbriae themselves or by fimbriae-dependent delivery of other host activating molecules such as lipopolysaccharide (LPS). Although LPS activates systemic inflammation through the CD14 and Toll-like receptor 4 (TLR4) pathways, mechanisms of epithelial cell activation by LPS are not well understood. These cells lack CD14 receptors and are unresponsive to pure LPS, but fimbriated Escherichia coli overcome this refractoriness and trigger epithelial cytokine responses. We now show that type 1 fimbriae can present an LPS- and TLR4-dependent signal to the CD14-negative epithelial cells. Human uroepithelial cells were shown to express TLR4, and type 1 fimbriated E. coli strains triggered an LPS-dependent response in those cells. A similar LPS- and fimbriae-dependent response was observed in the urinary tract of TLR4-proficient mice, but not in TLR4-defective mice. The moderate inflammatory response in the TLR4-defective mice was fimbriae dependent but LPS independent. The results demonstrate that type 1 fimbriae present LPS to CD14-negative cells and that the TLR4 genotype determines this response despite the absence of CD14 on the target cells. The results illustrate how the host "sees" LPS and other microbial products not as purified molecules but as complexes, and that fimbriae determine the molecular context in which LPS is presented to host cells.

Interleukin-8 receptor knockout mice have subepithelial neutrophil entrapment and renal scarring following acute pyelonephritis

Interleukin (IL)-8 receptor knockout (KO) mice were shown to have a dysfunctional neutrophil response to urinary tract infection and to develop renal scarring. Intravesical Escherichia coli infection stimulated epithelial chemokine secretion and IL-8 receptor expression in control mice. Neutrophils migrated through the tissues and crossed the epithelial barrier into the urinary tract lumen. In murine IL-8 receptor homologue (mIL-8Rh) KO mice, infection triggered a chemokine response, and neutrophils were recruited but failed to traverse the mucosal barrier and accumulated under the epithelium. After 7 days, control mice were healthy, and infection was cleared, but mIL-8Rh KO mice had swollen kidneys, with neutrophil abscesses and high numbers of bacteria. After 35 days, they developed kidney pathology and renal scarring. The results demonstrate that chemokine receptors drive transepithelial neutrophil migration. In their absence, the neutrophils are trapped, and the tissues are destroyed. This molecular deficiency may determine the progression from acute pyelonephritis to renal scarring.

Transepithelial neutrophil migration is CXCR1 dependent in vitro and is defective in IL-8 receptor knockout mice

Neutrophil migration across infected mucosal surfaces is chemokine dependent, but the role of chemokine receptors has not been investigated. In this study, chemokine receptors were shown to be expressed by epithelial cells lining the urinary tract, and to play an essential role for neutrophil migration across the mucosal barrier. Uroepithelial CXCR1 and CXCR2 expression was detected in human urinary tract biopsies, and in vitro infection of human uroepithelial cell lines caused a dramatic increase in both receptors. As a consequence, there was higher binding of IL-8 to the cells and the IL-8-dependent neutrophil migration across the infected epithelial cell layers was enhanced. Abs to IL-8 or to the CXCR1 receptor inhibited this increase by 60% (p<0.004), but anti-CXCR2 Abs had no effect, suggesting that CXCR1 was the more essential receptor in this process. Similar observations were made in the mouse urinary tract, where experimental infection stimulated epithelial expression of the murine IL-8 receptor, followed by a rapid flux of neutrophils into the lumen. IL-8 receptor knockout mice, in contrast, failed to express the receptor, their neutrophils were unable to cross the epithelial barrier, and accumulated in massive numbers in the tissues. These results demonstrate that epithelial cells express CXC receptors and that infection increases receptor expression. Furthermore, we show that CXCR1 is required for neutrophil migration across infected epithelial cell layers in vitro, and that the murine IL-8 receptor is needed for neutrophils to cross the infected mucosa of the urinary tract in vivo.

Interleukin 8 receptor deficiency confers susceptibility to acute experimental pyelonephritis and may have a human counterpart

Neutrophils migrate to infected mucosal sites that they protect against invading pathogens. Their interaction with the epithelial barrier is controlled by CXC chemokines and by their receptors. This study examined the change in susceptibility to urinary tract infection (UTI) after deletion of the murine interleukin 8 receptor homologue (mIL-8Rh). Experimental UTIs in control mice stimulated an epithelial chemokine response and increased chemokine receptor expression. Neutrophils migrated through the tissues to the epithelial barrier that they crossed into the lumen, and the mice developed pyuria. In mIL-8Rh knockout (KO) mice, the chemokine response was intact, but the epithelial cells failed to express IL-8R, and neutrophils accumulated in the tissues. The KO mice were unable to clear bacteria from kidneys and bladders and developed bacteremia and symptoms of systemic disease, but control mice were fully resistant to infection. The experimental UTI model demonstrated that IL-8R-dependent mechanisms control the urinary tract defense, and that neutrophils are essential host effector cells. Patients prone to acute pyelonephritis also showed low CXC chemokine receptor 1 expression compared with age-matched controls, suggesting that chemokine receptor expression may also influence the susceptibility to UTIs in humans. The results provide a first molecular clue to disease susceptibility of patients prone to acute pyelonephritis.

Neutrophil recruitment and resistance to urinary tract infection

This study examined the role of neutrophil leukocytes for the antibacterial defense at mucosal infection sites. Urinary tract infection (UTI) was established by injection into the bladder lumen of Escherichia coli 1177, a fully virulent clinical isolate. Infection of C3H/HeN (lpsn, lpsn) mice recruited neutrophils into the urinary tract, and bacteria were cleared from kidneys and bladders. The neutrophil response was absent in C3H/HeJ (lpsd, lpsd) mice, and bacteria persisted in the tissues. Peripheral neutrophil depletion of C3H/HeN mice was subsequently achieved by pretreatment with the granulocyte-specific antibody RB6-8C5. The E. coli-induced neutrophil recruitment was inhibited, as shown by immunohistochemistry and tissue myeloperoxidase quantitation. As a consequence, bacterial clearance from kidneys and bladders was drastically impaired. Antibody treatment of C3H/HeJ mice had only a marginal effect. The results show that neutrophils are essential for bacterial clearance from the urinary tract and that the neutrophil recruitment deficiency in C3H/HeJ mice explains their susceptibility to gram-negative UTI.

Role of fimbriae-mediated adherence for neutrophil migration across Escherichia coli-infected epithelial cell layers

This study examined the role of P and type 1 fimbriae for neutrophil migration across Escherichia coli-infected uroepithelial cell layers in vitro and for neutrophil recruitment to the urinary tract in vivo. Recombinant E. coli K-12 strains differing in P or type 1 fimbrial expression were used to infect confluent epithelial layers on the underside of transwell inserts. Neutrophils were added to the upper well, and their passage across the epithelial cell layers was quantified. Infection with the P- and type 1-fimbriated recombinant E. coli strains stimulated neutrophil migration to the same extent as a fully virulent clinical E. coli isolate, but the isogenic non-fimbriated vector control strains had no stimulatory effect. The enhancement of neutrophil migration was adhesion dependent; it was inhibited by soluble receptor analogues blocking the binding of P fimbriae to the globoseries of glycosphingolipids or of type 1 fimbriae to mannosylated glycoprotein receptors. P- and type 1-fimbriated E. coli triggered higher interleukin (IL) 8 secretion and expression of functional IL-8 receptors than non-fimbriated controls, and the increase in neutrophil migration across infected cell layers was inhibited by anti-IL-8 antibodies. In a mouse infection model, P- or type 1-fimbriated E. coli stimulated higher chemokine (MIP-2) and neutrophil responses than the non-fimbriated vector controls. The results demonstrated that transformation with the pap or fim DNA sequences is sufficient to convert an E. coli K-12 strain to a host response inducer, and that fimbriation enhances neutrophil recruitment in vitro and in vivo. Epithelial chemokine production provides a molecular link between the fimbriated bacteria that adhere to epithelial cells and tissue inflammation.