<p>Urinary tract infections (UTIs) are the second most common type of infection identified in the clinical setting and disproportionately afflict women. UTIs most frequently manifest in the form of infection of the lower urinary tract, involving the bladder. Uropathogens, particularly uropathogenic E. coli, progressively colonize the urethra and ascend to the bladder, where they initiate cystitis. In some cases, infection further ascends through the ureters and reaches the kidneys, where it causes pyelonephritis. Infection of both the upper and lower urinary tract can have serious ramifications for the host, and this is in large part due not to infection itself but to host-directed responses to bacterial insults. </p><p> In this thesis, I will describe and discuss two distinct aspects of UTIs. In the first study, in vivo work in a mouse model of urinary tract infection revealed a novel role for mast cells, which are tissue-resident granulated innate immune cells, in directing the detachment and death of epithelial cells during cystitis, facilitating the clearance of bacteria from the bladder. An ex vivo porcine bladder infection model suggested a specific role for mast cell granules and the proteases contained therein, which was corroborated with in vitro experiments utlizing isolated mast cell granules and human epithelial cells to demonstrate granule-induced exfoliation and cell death. From this work, it is clear that mast cells play a highly targeted role in modulating urothelial integrity during bladder infection by mediating host-directed epithelial loss.</p><p> In the second study described in this dissertation, the synergistic roles of both pyelonephritis and vesico-ureteric reflux (VUR), a congenital urinary tract defect that results in the improper backflow of urine from the bladder to the kidney, in the development of reflux nephropathy, a fibrotic host response characterized by renal scar formation, were elucidated in a series of in vivo experiments. Specifically, the C3H mouse, which is naturally susceptible to VUR, was utilized to characterize the dynamics of kidney infection and the onset of reflux nephropathy. Renal scarring was dependent on the presence of sustained kidney infection and the accompanying inflammatory response due to VUR, while neither transient infection nor reflux alone were sufficient to provoke nephropathy. Thus, the development of reflux nephropathy is dependent upon the confluence of both infection and VUR. </p><p> This body of work reveals the double-edged sword of the host inflammatory response to urinary tract infection. In the bladder, mast cell activation and degranulation leads to granule-induced epithelial exfoliation and consequently a reduction in the bacterial burden in the bladder. However, the sustained inflammatory response that accompanies pyelonephritis in vesico-ureteric reflux-affected individuals results in significant damage to the kidney without any accompanying reduction in infection. These findings highlight the dueling roles of the host inflammatory response to infection in the upper and lower urinary tract and strongly suggest that differential clinical approaches to cystitis and pyelonephritis are necessary to promote an effective mast cell in the bladder in the former and facilitate the clearance of renal infection while mitigating tissue damage in the latter.</p> / Dissertation