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CUTTING EDGE |



* Institute for Systems Biology, Seattle, WA 98103; Department of
Immunology,
Department of Medicine, and
Department of Pathology, University of Washington School of Medicine, Seattle, WA 98195; and
¶ Department of Host Defense, Research Institute for Microbial Diseases, Osaka University, Osaka, Japan
| Abstract |
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| Introduction |
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Many important pathogenic bacteria, both Gram-positive and Gram-negative, are flagellated. Flagellated uropathogenic E. coli (UPEC)4 cause 7090% of all urinary tract infections (UTI), and their pathogenesis involves contact between bacteria and the epithelial cell surface of the urogenital tract, a site of TLR5 expression in humans (6). UPEC colonize the urethra and ascend to the bladder, where they can persist at high levels (7). In addition to cystitis in the bladder, UPEC may ascend to the kidney and cause serious complications, including pyelonephritis and bacteremia (8). E. coli is recognized by several TLRs, including TLRs 2, 4, 5, and 11, and likely also TLR9. Previous studies indicate that TLR4 and TLR11 regulate susceptibility to UTIs (9, 10, 11). However, it is not currently known whether TLR5 is critical for host defense to UTIs or whether there is sufficient TLR redundancy to obviate its requirement.
| Materials and Methods |
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Tlr5/ mice (strain designation B6.129P2-Tlr5tm1Aki) were derived and backcrossed to a C57BL/6 background for eight generations as previously described (5). Wild-type (WT) control mice were from a C57BL/6 background (The Jackson Laboratory). E. coli strain CFT073, from a patient with acute pyelonephritis (American Type Culture Collection), was grown in Luria-Bertani (LB) medium in static culture at 37°C for 48 h. Expression of type 1 pili was confirmed for each experiment by testing for yeast agglutination (12). Flagellin was purified from S. typhimurium as described in Ref. 3 and was heated to 70°C for 15 min to monomerize it. Contaminating endotoxin was removed by passage through a 100-kDa molecular mass cutoff filter (Millipore) followed by endotoxin removal on a polymixin B column (Pierce). The resulting flagellin did not show detectable endotoxin by Limulus assay (Cambrex). Ultrapure LPS was purchased from List Biologicals.
Real-time PCR
RNA was extracted from organs with TRIzol (Invitrogen Life Technologies), DNase treated with TURBO DNA-free (Ambion), and cDNA produced with Superscript II (Invitrogen Life Technologies). Real-time PCR was performed with TaqMan Fast (Applied Biosystems) on an Applied Biosystems Prism 7900 HT. Primer/probe sets for elongation factor 1
(EF1
) were designed with Primer Express 1.0 (PerkinElmer) with a 5'-FAM and 3'-TAMRA modification (Biosearch Technologies). Primer/probe sets for mouse TLR5 and cytokines/chemokines were purchased from Applied Biosystems. Threshold cycle (Ct) values were transformed by 1/2Ct, and then normalized to EF1
for each organ.
NF-
B luciferase reporter assay
Chinese hamster ovary (CHO) K1 cells stably expressing mouse TLR5 and NF-
B luciferase reporter constructs (3) were stimulated with heat-killed bacteria for 4 h and assayed for luciferase activity. Assays were done in duplicate, and the experiment was repeated three times. Percent fold induction was calculated by dividing the luciferase values for each bacterial dose by the maximal luciferase value for the bacteria in each experiment.
UTI model of infection
The Institute for Systems Biology and Osaka University Institutional Animal Care and Use Committees approved all animal protocols. Forty-eight-hour static cultures of E. coli CFT073 were resuspended in cold PBS at 1 x 109 CFU/ml. Anesthetized mice were inoculated transurethrally with 5 x 107 E. coli in 50 µl, and urethras were coated with collodion (Sigma-Aldrich) (13). Six hours postinfection, collodion was removed by blotting with acetone. At each time point, organs were homogenized in 1 ml of 0.025% Triton X-100/PBS and plated on LB-agar to enumerate CFUs.
Statistical analysis
Comparisons were made with a two-tailed Mann-Whitney U test or a Students t test. A p
0.05 was considered to be significant. Statistics were calculated with PRISM4 (GraphPad).
Histology
Bladders and kidneys were fixed in 10% formalin-buffered saline and embedded in paraffin. Four-micrometer sections were cut, stained with H&E, and examined by a pathologist blinded to mouse genotype.
| Results and Discussion |
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To test our hypothesis that TLR5 is critical for host defense against E. coli UTIs, we first examined bladder and kidney tissue for TLR5 expression. We extracted RNA from tissues of C57BL/6 mice and evaluated expression levels by real-time PCR (Fig. 1A). TLR5 was expressed in both bladder and kidney, which suggested that it may regulate critical aspects of the immune response during UTI. We next examined whether Tlr5/ mice were more susceptible to urinary tract infections. When grown in static culture, uropathogenic E. coli forms type 1 pili (12) that enhance adherence to bladder epithelia and increase bladder colonization. We first determined whether growth in static culture resulted in flagellin expression. CHO cells stably expressing mouse TLR5 and a NF-
B-dependent luciferase reporter construct responded to statically grown heat-killed bacteria in a dose-dependent manner, detecting fewer than 8000 bacteria, a multiplicity of infection of
0.1 (Fig. 1B). Control CHO cells expressing the pEF6 vector alone did not respond to bacteria (data not shown).
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We next examined whether the E. coli disseminated to the kidney. Bacteria were present in the kidneys of both WT and TLR5-deficient animals, and no significant CFU differences were observed between WT and Tlr5/ mice in the kidney at early time points (Fig. 1D). By day 5 after infection, however, no bacteria were detected in WT kidneys, but Tlr5/ mice had a median of 6.5 x 102 CFU/kidney (IQR: 0- 6.5 x 104, p = 0.0274 by Mann-Whitney U test) with six of nine infected mice showing kidney counts (Fig. 1D). Together, these data suggest that Tlr5/ mice are unable to control bacterial replication and cannot clear the infection from the kidneys by day 5.
Tlr5/ mice exhibit decreased inflammation at 2 days postinfection
We next examined histologic sections from WT and knockout mice by light microscopy to determine the pathologic consequences of TLR5 deficiency. Bladders and kidneys from WT and Tlr5/ mice exhibited similar levels of inflammation at 4 and 24 h after infection (data not shown). In contrast, at 2 days postinfection, Tlr5/ mice showed decreased inflammation in the bladder relative to WT mice (Fig. 2A). WT mice exhibited prominent submucosal edema and infiltration of the submucosa and epithelium by leukocytes. There was no significant inflammation in the kidneys from WT or Tlr5/ mice at the 2-day time point (data not shown).
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Flagellin induces early expression of proinflammatory genes in the bladder
To identify proinflammatory molecules up-regulated by TLR5 in the bladder that might lead to increased early inflammation in WT mice, we examined the in vivo response to transurethral administration of flagellin and compared this with LPS, another prominent TLR agonist present in E. coli. Mice were inoculated transurethrally with 30 µg of flagellin or 10 µg of ultrapure LPS in PBS, and bladders were harvested at 4 h. Real-time PCR was performed on bladder tissue for several proinflammatory cytokines and chemokines. Transurethral inoculation of flagellin up-regulated expression of KC (CXCL1), MIP2 (CXCL2), MCP-1 (CCL2), IL-6, and TNF-
mRNA, but not
-defensin 1 mRNA in WT mice (Fig. 3). As expected, Tlr5/ mice did not respond to flagellin in the bladder. In contrast to flagellin, LPS delivered into the bladders of WT mice did not induce transcription of these proinflammatory genes. These results demonstrate that flagellin induces a robust TLR5-dependent innate immune response in the murine bladder that may account for its critical role in UTI pathogenesis.
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Bacterial motility is important for virulence in some models of UTI, which suggests that TLR5-flagellin interactions may be important for bacterial uropathogenesis. Flagellum-negative mutants of Proteus mirabilis are significantly less successful at bladder colonization and do not progress to the kidneys as readily (20). Furthermore, two recent studies suggest that flagellar mutants of E. coli are less able to colonize the mouse urinary tract (8, 14). Flagellar-based motility may be beneficial in early colonization of the urinary tract, but may not be required for maintenance of infection (8). This is in agreement with a study that demonstrated down-regulation of flagellin genes by E. coli CFT073 several days after in vivo infection (21). These studies combined with our results suggest that TLR5 recognition of flagellin is an important component of the innate immune response to E. coli during the early stages of UTI when flagellin expression and motility contribute to colonization of the urinary tract.
In addition to TLR5 recognition of extracellular bacterial flagellin, two novel intracellular flagellin receptors have recently been described that are both members of the nucleotide-binding oligomerization domain leucine-rich repeat family (22, 23, 24). Naip5 detects flagellin from Legionella pneumophila that reaches the macrophage cytosol via the bacterias type IV secretion system (24, 25). Ipaf detects cytoplasmic flagellin injected into macrophages by the type III secretion system of S. typhimurium (22). The roles of these additional flagellin receptors in UTI are not known, but the UPEC strain used in this study, like many other UPEC (26), does not encode a type III secretion system (27). Thus, intracellular flagellin receptors may not be able to compensate for the lack of flagellin recognition by TLR5 in this infection model.
Although TLR5 has been implicated in the innate immune response to mucosal infection (5, 28), there has been limited in vivo data to substantiate this claim. A recent study of Tlr5/ mice did not find a unique role for TLR5 in defense to Salmonella or Pseudomonas (29). Our study provides the first evidence that TLR5 regulates a critical and nonredundant role in the innate immune response to a murine infection with extracellular flagellated bacteria. We have previously identified a TLR5 polymorphism present in the general population that results in a stop codon that abrogates TLR5 signaling and is associated with increased susceptibility to Legionnaires disease (30). These murine studies support a hypothesis that individuals who possess this TLR5 variant will also be more susceptible to UTI.
| Acknowledgments |
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| Disclosures |
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| Footnotes |
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1 T.R.H. was supported by National Institutes of Health Grant R01 AI061464-02, K.D.S. was supported by National Institutes of Health Grant R01 AI062859, and A.A. was supported by National Institutes of Health Grant R01 AI052286. ![]()
2 E.A.-N. and T.R.H. contributed equally to this work. ![]()
3 Address correspondence and reprint requests to Dr. Alan Aderem, Institute for Systems Biology, 1441 North 34th Street, Seattle, WA 98103. E-mail address: aderem{at}systemsbiology.org ![]()
4 Abbreviations used in this paper: UPEC, uropathogenic E. coli; UTI, urinary tract infection; EF1
, elongation factor 1
; CHO, Chinese hamster ovary; LB, Luria-Bertani; IQR, interquartile range; WT, wild type. ![]()
Received for publication August 14, 2006. Accepted for publication February 20, 2007.
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via Ipaf. Nat. Immunol. :
in Salmonella-infected macrophages. Nat. Immunol. : This article has been cited by other articles:
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