Mucosal receptors involved in pathogen recognition and host defence
Sammanfattning: Urinary tract infection is the most common infection in man. P fimbriae are virulence factors of uro-pathogenic Escherichia coli. They are expressed by 70-90 % of acute pyelonephritis isolates and up to 100% of the most virulent isolates that cause bacteremia. This thesis concerns the interaction between P fimbriae and the urinary tract mucosa, and the host receptors involved in fimbrial recognition and trans membrane signalling. 1. Recognition receptors. P fimbriae recognise receptor epitopes in cell surface glycosphingolipids. When the receptor expression was inhibited with the synthetic ceramide analogue PDMP, attachment was inhibited, as was the cellular response to infection, demonstrating that these receptors are essential for the initial tissue attack by the bacteria. Similar results were obtained using the natural iminosugar NB-DNJ, which blocks the synthesis of ceramide-linked oligosaccharides. In vivo inhibition of the UDP-glucose-glycosyltransferase was shown to cause receptor depletion on the uroepithelial cells and the P fimbriated E. coli failed to colonize the treated mice or to evoke an inflammatory response. The results confirmed the importance of glycosphingolipid receptors for fimbriae, and suggested that NB-DNJ might be explored as a therapeutic alternative in the treatment of UTI. 2. TLR4 and the innate host response. Transmembrane signalling in response to P fimbriated E. coli was shown to depend on TLR4. TLR4 KO mice became chronically infected, and were unable to clear an experimental infection. The results showed that a single host gene can determine the quality of the mucosal response and that this response determines if infection will become symptomatic or asymptomatic. 3. Adaptor protein usage. Two groups of adaptor proteins are involved in TLR4 signalling. The MyD88 /TIRAP adaptors control the CD14 dependent response to LPS/LBP, while TRIF/TRAM have other functions. This study showed that trif/tram controls the CD14 independent cell activation by P fimbriated E. coli. Mice lacking these adaptors developed acute symptoms and were unable to clear the infection. The LPS related Myd88 and TIRAP adaptors, in contrast, were not required to activate the innate defence, but they were needed for efficient clearance of infection. Defects in the TRIF/TRAM pathway may explain the tendency of certain individuals to develop asympromatic bacteriurea rather than symptomatic infection. 4. Ceramide as a signalling intermediate. Following the binding of P fimbriae, the glycosphingolipid receptors are cleaved and ceramide is released. Ceramide has previously been recognised as a signalling intermediate in the sphingomyelin pathway. The results of this study demonstrate that ceramide can act as a signalling intermediate between the recognition receptor and TLR4, thus providing a mechanism for the recruitment of TLR4 by P fimbriated bacteria. 5. Renal scarring in mIL8Rh KO mice. Experimental UTI in mIL-8Rh deficient mice identified the first genetic defect explaining the increased susceptibility to acute pyelonephritis and renal scarring in certain individuals. The IL-8 receptor deficiency caused massive neutrophil trapping in the kidney with tissue damage resembling renal scarring. The mIL-8Rh deficient mouse thus offers a highly relevant model to study the cellular and molecular mechanisms that underlie the development of renal scarring.
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