Murine models of UTI in young na
Murine models of UTI in young naïve mice have elucidated critical details of acute UPEC pathogenesis, involving the invasion of UPEC into bladder epithelial (urothelial) cells (Hannan et al., 2012; Brumbaugh and Mobley, 2012). Internalized UPEC are able to avoid a TLR4-mediated exocytic process (Song et al., 2009) and escape into the host cell cytoplasm, where they replicate into biofilm-like intracellular bacterial communities (IBCs) (Justice et al., 2004; Anderson et al., 2003). IBCs are routinely observed in urine cytology of individuals presenting with UTI, supporting the validity of their importance in pathogenesis and the ability of the mouse model to recapitulate human disease (Rosen et al., 2007; Robino et al., 2013; Robino et al., 2014). This process allows UPEC to establish infection and persist in the face of a stringent population bottleneck (Hannan et al., 2012; Schwartz et al., 2011) caused by the host\'s acute multi-prong defense: including secretion of cytokines (Duell et al., 2012; Ingersoll et al., 2008; Ragnarsdottir et al., 2011), activation and infiltration of immune cells (Haraoka et al., 1999; Schiwon et al., 2014; Chan and St John, 2013), and exfoliation of epithelial cells (Mulvey et al., 1998; Dhakal and Mulvey, 2012). Exactly how these host responses act in a coordinated fashion to clear infection, how a multitude of UPEC virulence factors act to promote infection, and how bacterial and host factors interact to determine disease outcome and susceptibility to recurrent UTI (rUTI) are poorly understood.
There are two main outcomes of UPEC bladder infection in naïve mice: i) sterilization of the urine within days of acute infection with or without the establishment of a quiescent intracellular reservoir (Mysorekar and Hultgren, 2006; Mulvey et al., 2001), or ii) persistent high titer bacteriuria and chronic high titer bladder infection with chronic bladder inflammation (chronic bacterial cystitis) that lasts for the lifetime of the animal if not cleared by appropriate direct thrombin inhibitor (Hannan et al., 2010). Which of these outcomes occurs after UPEC infection in C3H/HeN mice is determined within the first 24h post-inoculation (hpi) and depends on the severity of the host\'s acute inflammatory response (Hannan et al., 2010). Specifically, severe pyuria and bladder inflammation with elevated serum interleukin-5 (IL-5) and serum and urine IL-6, the neutrophil chemokine CXCL1, and granulocyte colony-stimulating factor (G-CSF or CSF3) at 24hpi are predictive of chronic infection. Whether chronic cystitis in mice is analogous to an untreated clinical chronic symptomatic UTI or an acute symptomatic UTI that resolves into asymptomatic bacteriuria (ASB) is not clear, but in contrast to immunodeficient mouse models of ASB (Ragnarsdottir et al., 2011) chronic cystitis in immunocompetent mice results from ongoing extracellular bacterial replication on the inflamed bladder mucosa in the face of a robust neutrophil response. This chronic bladder inflammation manifests as both lymphonodular hyperplasia in the bladder submucosa and urothelial hyperplasia, with a lack of uroplakin expression, a marker for terminal differentiation, in superficial facet cells (Hannan et al., 2010). Similar histological findings have been observed in humans suffering persistent bacteriuria and recurrent UTI (Schlager et al., 2011; Hansson et al., 1990). Significantly, chronic bladder inflammation in mice appears to cause mucosal remodeling that renders the bladder more susceptible to UTI upon further bacterial challenge weeks after resolution of the primary infection with antibiotic therapy, suggesting that this provides a clinically relevant model for rUTI (Hannan et al., 2010).
Materials and Methods
Discussion The rapid emergence of multidrug resistant UPEC clones, including strains that express carbapenemases, highlights the urgent need for alternatives for treatment and prevention of rUTI. Here, we provide evidence that high levels of neutrophil-mediated damage within the urothelial barrier contribute to host susceptibility to rUTI. Neutrophil-mediated mucosal damage has been demonstrated to exacerbate infection in other models, including both influenza and group A streptococcal infections of the respiratory tract (Brandes et al., 2013; Bhowmick et al., 2013). Consistent with this, we identified increased serum levels of chemokines and growth factors involved in myeloid cell inflammation during an initial UTI as biomarkers for increased susceptibility to rUTI within three months, particularly in patients for whom the initial UTI was their first lifetime UTI. We found that inflammatory monocyte recruitment to the infected bladder was not absolutely necessary for urothelial transmigration by neutrophils in contrast to results in a recent study of UTI using C57BL/6J mice (Schiwon et al., 2014). Our data suggest that bladder resident macrophage responses and COX-2 signaling compensate for the loss of inflammatory monocyte recruitment in C3H/HeN mice. Further, our proteomics study demonstrated a global reorganization of urothelial markers in convalescent mice with a history of chronic cystitis (“sensitized”). This apparent mucosal remodeling is consistent with our hypothesis that upon bacterial challenge the bladders of sensitized mice are prone to more severe inflammation and mucosal damage from proteases secreted by neutrophils and other immune cells. Overall, these findings set the stage for targeted translational studies of these proteins in clinical specimens.