Molecular determinants of disease severity in urinary tract infection

University dissertation from Lund University, Faculty of Medicine

Abstract: In the urinary tract, Escherichia coli infection may result in life-threatening disease, or asymptomatic bacterial carriage, comparable to bacterial commensalism in the gut. Pathogenic strains trigger a disease provoking host response which differs depending on the infected organ. The diversity of the response reflects the virulence repertoire of the infecting strain and by the susceptibility of the host. During asymptomatic bacteriuria (ABU), the lack of clinical symptoms has been attributed to the loss of virulence by the strains. ABU strains have a reduced genome size and carry point mutations or deletions in virulence genes. In the prototype strain E. coli 83972 fimbrial gene clusters are affected by multiple point mutations (the papG adhesin gene) or large deletions (the fim gene cluster). As a result, the innate immune response to ABU strains is reduced. In addition, we made the discovery that asymptomatic carrier strains actively inhibit RNA Polymerase II (Pol II) Ser2 phosphorylation. Pol II is the enzyme required for the transcription of all mRNA in eukaryotes and its function is tightly regulated. E. coli 83972 suppresses Pol II-dependent host gene expression, including genes involved in immune activation. Compared to a broad chemical inhibitor, DRB, inhibition was more limited, suggesting a degree of specificity for a certain repertoire of host genes. Through this suppression of transcription, ABU strains may promote tissue integrity while inhibiting potentially destructive immune activation. NlpD was identified as a protein released by asymptomatic carrier strains with potent host gene expression inhibitory capacity. NlpD targeted the Pol II phosphorylation machinery by interacting with the biggest Pol II subunit, RPB1, and PAF1C in host cells. In treated mice, NlpD inhibited the destructive arm of innate immune activation and reduced bacterial loads. Our findings suggest that molecules of bacterial origin may be explored as therapies to reproduce the beneficial effects associated with ABU.The symptoms of acute cystitis, in contrast, are caused by an exaggerated inflammatory response triggered by infection. We now define acute cystitis as an IL-1β-driven, hyper-inflammatory disorder with atypical IL-1β processing through MMP-7. The results also suggest a genetic susceptibility factor in patients with severe and chronic cystitis, through mutations affecting ASC or NLRP3 expression. ASC and NLRP-3 were identified as negative regulators of MMP-7 expression and the identified molecular determinants and IL-1β and MMP-7 as novel targets for immunomodulatory therapy with potent effects in vivo as well as biomarkers for acute cystitis.In contrast to acute cystitis, acute pyelonephritis is a severe, sometimes life-threatening kidney infection with systemic involvement and risk of developing bacteremia. Uropathogenic E. coli strains initiate tissue attack of the renal pelvis thus starting disease pathogenesis. A normally protective innate immune signalling cascade, controlled by toll like receptor (TLR)4, is exaggerated explaining the acute disease and sometimes destructive, long-term effects. The response is determined, in part, by bacterial P fimbriae, their receptors and the quality of the signalling cascade that they activate, including several transcription factors. The transcription factor IRF-3 controls the protective arm of the innate immune response to kidney infection, and as a result, mice lacking Irf3 develop severe infection accompanied by urosepsis and renal abscess formation. We identify IRF-7 as a driving force for the disease response in Irf3-/- mice and development of renal pathology. We also define Irf7 as an immuno-therapeutic target that can be controlled with small interfering RNA (siRNA) to restore the balance of resistance versus pathology and prevent kidney damage.Finally, we show that P fimbriae influence the IRF-7 expression and the repertoire of downstream genes associated with acute pyelonephritis (APN). Specifically, P fimbriae were shown to act as IRF-7 agonists. The expression of functional P fimbriae was sufficient to reprogram host gene expression, through effects of PapG on the transcriptional machinery of the host. Paradoxically, the results suggest that a single “super virulence” factor may be sufficient to tilt the balance from peaceful coexistence to disease and for the host to recognize and respond to a strain that lacks most other virulence factors.

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