The established model of transurethral catheterization of mice allows the study of bladder pathologies, including urinary tract infection, but can only be performed in females. A new model of male transurethral instillation, presented here, will enable research in an area marked by strong clinical and epidemiological differences between the sexes.
Urinary tract infections (UTI) are extremely common worldwide, incurring significant morbidity and healthcare-associated expenses. Small animal models, which accurately reflect disease establishment and progression, permit dissection of host-pathogen interactions and generation of immunity to infection. In mice, intravesical instillation of uropathogenic E. coli, the causative agent in more than 85% of community acquired UTI, recapitulates many of the stages of infection observed in humans. Until recently, however, UTI could only be modeled in female animals. This limitation has hindered the study of sex-related differences in UTI, as well as other bladder pathologies, such as cancer. Here, we describe a method to instill male mice that allows direct comparison between female and male animals and provide a detailed protocol to assess bladder tissue by flow cytometry as a means to better understand host responses to infection. Together, these approaches will aid in the identification of host factors that contribute to sex biases observed in UTI and other bladder-associated diseases.
Urinary tract infections (UTI) are one of the most common infections in developed countries1. Infection rates are similar between females and males among neonates and the elderly2. Premenopausal adult women, however, have a greatly increased incidence of community-acquired UTI compared to men2,3. Given that this disease primarily impacts women, fundamental and clinical research has overwhelmingly focused on UTI in females. However, UTI in men is a significant and understudied health care challenge4. Indeed, because UTI in men are associated with higher morbidity, these infections are routinely defined as complicated4,5.
As our understanding of the central role of sex biases in physiology and pathology evolves, new methods are required to explore this previously neglected aspect of disease. Sex differences play an essential role in immunity and infection; females have higher incidence of autoimmune disease, while males are more susceptible to certain infections, such as tuberculosis, malaria, and HIV6,7. Bladder cancer, another urologic pathology, is significantly more prevalent in men than in women, and several studies have shown a role for androgens in the development of malignancy8,9,10,11,12. Notably, however, investigation of intravesical therapies for bladder cancer is performed exclusively in female animals, due to the inability to repeatedly catheterize male mice13.
The study of UTI pathogenesis relies heavily on rodent models of infection (e.g., mice and rats). Murine models of UTI may employ uropathogens originally isolated from human infections, such as uropathogenic E. coli (UPEC), Klebsiella, Enterococcus, Staphylococcus, or Proteus14. Typically, bacteria are introduced into the bladder via catheterization of the urethra. Following infection, bladders and kidneys can be removed to assess specific parameters of infection, such as bacterial colonization, tissue damage, or host immune response15,16. Universally, however, only female animals are used for UTI research. Indeed, many studies have noted that, due to anatomical reasons, catheterization of male mice is not possible13,17,18,19. More recently, a surgical approach to male instillation has been described, in which the abdomen is opened, the bladder is externally displaced by applying gentle pressure to the opening in the abdomen, and bacteria are injected into the bladder20. This approach enables male infection, at the cost of surgical intervention. Thus, a major caveat of this approach includes the influence of inflammation, on the outcome of infection such as the potential to induce an anti-inflammatory wound-healing response to the incision21. As our interests include understanding sex bias in response to disease, we developed a method of bacterial intravesical instillation in male mice that more closely matches the long-established non-surgical transurethral approach used in female rodents22.
Our model builds upon an established methodology and provides the ability to directly compare the host immune response to UTI in female and male animals. This method will permit dissection of sex-based differences in infection, and potentially offer molecular and cellular clues to the pronounced differences in susceptibility and response to infection between the sexes. Additionally, this model has value beyond UTI studies, allowing the establishment of models to investigate other bladder-associated diseases, such as bladder cancer, prostatitis, under- or over-active bladder syndrome, and interstitial cystitis.
Mouse experiments were conducted in accordance with approval of protocol number 2012-0024 by the Comité d'éthique en expérimentation animale Paris Centre et Sud (the ethics committee for animal experimentation), in application of the European Directive 2010/63 EU.
1. Preparation of Catheters
2. Preparation of Uropathogenic E. coli for Infection
3. Preparation of Mice
4. Transurethral Instillation of Female Mice
5. Transurethral Instillation of Male Mice
6. Determination of CFU in Infected Organs
7. Flow Cytometric Analysis of Bladder Tissue
In the development of this protocol, cohorts of female and male C57Bl/6 mice aged 6 to 8 weeks were instilled, and bacterial burden evaluated in bladders at time points ranging from 1 hr to 30 days. The results from these studies are detailed elsewhere (Zychlinsky Scharff et al., pending). Here, we present representative data from 24 hr infections. Notably, bacterial burden was equivalent between male and female mice at 24 hr post-infection (Figure 1). Little variation in bacterial burden was observed within each group 24 hr post-infection, similar to what has been reported in female mice15,16.
If care is taken to sufficiently empty bladders of urine prior to infection, significant leakage of the inoculum is rarely observed in female mice. However, instillation into male mice resulted in significant amounts of bacterial inoculum leaking from the urethra, particularly during the development stage of this protocol. To determine whether loss of inoculum at the time of infection impacted colonization or the establishment of infection, we employed a rating system for each instillation. Each instillation was scored on a scale of 1 – 5, with 1 being the most optimal, and bacterial colonization was determined 24 hr post-infection (Figure 2, box). While this system was investigator dependent, and therefore, potentially subjective, the primary author of this study determined all scores immediately following instillation, prior to evaluation of bacterial burden in infected bladders. No statistically significant differences existed among the CFU obtained from animals with different instillation scores, as assessed by a nonparametric Kruskal-Wallis test comparing the mean rank of each column with the mean rank of every other column (p = 0.17) and correcting for multiple comparisons using a Dunn's test. From this analysis, it can be concluded that suboptimal instillations (score >3), resulting in substantial leakage of bacterial inoculum during infection, do not significantly impact bacterial colonization at 24 hr (Figure 2, graph). Notably, with experience, the frequency of leaking in male mice diminished significantly.
Finally, the objective in developing this model was to directly compare the immune response to UPEC in female and male animals. To test whether cellular infiltration is altered between the sexes in response to UTI, the number of CD45+ immune cells in naïve and infected cohorts of female and male mice were assessed by flow cytometry. While the number of immune cells present in naïve animals was not different between female and male mice (p = 0.95), there was a statistically significant increase in infiltration into the bladders of infected female mice (p = 0.0015) (Figure 3A-B).
Figure 1: UPEC Colonizes Female and Male Bladders with Equal Efficiency. Six to 8 week old female and male C57Bl/6 mice were instilled with 107 CFU of UPEC. 24 hr post-infection, bladders were aseptically removed to enumerate bacterial burden. Plot depicts CFU/bladder. Each dot is one mouse, representative experiment of 5 shown. Please click here to view a larger version of this figure.
Figure 2: The Quality of Instillation does not Correlate with Bacterial Burden at 24 hr Post-infection. Six to 8 week old female and male C57Bl/6 mice were instilled with 107 CFU of UPEC. Immediately after each instillation, a single researcher assigned a quality score to the instillation, as defined by specific criteria (boxed text). 24 hr post-infection, bladders were aseptically removed to enumerate bacterial burden. Plot depicts the assigned quality score versus CFU/bladder. Each dot is one mouse, 5 pooled experiments are shown. p = 0.17, Kruskal-Wallis test comparing the mean rank of each column with the mean rank of every other column with post hoc Dunn's test for multiple comparisons. Please click here to view a larger version of this figure.
Figure 3: Immune Cell Infiltration is Increased in Female Mice in Response to UTI. Six to 8 week old female and male C57Bl/6 mice were instilled or not with 107 CFU of UPEC. Representative dot plots depict total bladder cells with CD45+ immune cells gated in pink from naïve or infected mice. Graph shows the absolute number of CD45+ immune cells in bladders from naïve or 24 hr infected mice. Each dot is one mouse, 2 pooled experiments are shown. p-values determined by Mann-Whitney test. Please click here to view a larger version of this figure.
Transurethral instillation of male mice offers many new research opportunities into the influence of sex on bladder mucosal disease, but also presents several challenges. Foremost, one limitation is that the instillation may initially prove to be technically difficult, resulting in excessive inflammation during catheter insertion. Improvement in technique can be achieved by the instillation of dead mice with a colored solution, such as Evans blue dye. To confirm that the instillation is successful, bladders should be visually inspected and the volume of instilled liquid can be evaluated by extraction with a tuberculin syringe. The importance of slow, gentle movements cannot be overstated: there should be no resistance and no force used to insert the catheter, as this will result in excess inflammation and tissue damage. When performed correctly, the catheter will slide effortlessly into the urethra. If resistance or obstruction is felt, it is best to remove the catheter and reattempt insertion.
By correlating the quality of the instillation to bacterial burden 24 hr post-infection, we demonstrated that an imperfect technique, in which a small amount of the bacterial inoculum is lost at the time of infection, still results in robust colonization of the bladder. In utilizing this protocol, an investigator should aim to achieve leak-free instillations. However, the robustness of the procedure ensures that imperfect instillations will still provide useful data and interpretable results.
The great advantage of our technique over the only published alternative method of male bladder infection is its non-invasive, non-surgical approach. Our approach using transurethral instillation follows the physiological route through the urethra to the bladder, without disrupting the structural integrity of the abdomen. The invasive technique, described by Olson and colleagues20, includes factors inherent to surgical procedures, including inflammation, delayed healing, and resultant scar tissue. Especially relevant with regard to studies of immune response to infection is the organism's response to surgical trauma. This includes the formation of a pro-inflammatory milieu and potential tissue granulation, as well as an anti-inflammatory wound healing program as part of the healing process. These factors represent undesirable influences within the experimental setting.
Finally, the objective of this study was to develop a method that allows direct comparison of male and female immune responses during the course of UTI, which can be applied to future studies addressing the influence of sex on bladder mucosal disease, such as infection and cancer biology. As a second distinct advantage, the method described here mirrors that used in female mice for decades19. Thus, experiments performed in male mice with this technique are comparable to a wide body of existing research. Our experiments have revealed that differences in response do exist and that these differences may provide clues to understanding sex-based disparities in response to mucosal infections, as well as other diseases of the bladder.
The authors have nothing to disclose.
We thank Dr Matthieu Rousseau for critical reading of the manuscript and the Laboratory of Dendritic Immunobiology for their helpful insights during the development of this protocol and project. This work was supported in part by funding from the European Union Seventh Framework Programme Marie Curie Action (PCIG11-GA- 2012-3221170, and the Immuno-Oncology LabEx (MAI).
BD Insyte Autoguard Shielded IV Catheters 24G, 0.7 mm external diameter, 14 mM long | BD Medical | 381811 or 381411* | * catalog number is country-dependent |
inoculating loop | Greiner Bio-One | 731171 | |
LB Miller broth | Difco | 244620 | |
LB Miller agar | Difco | 244520 | |
Cuvettes | Bio-rad | 223-9955 | |
syringes | B Braun | 9166017V | |
Thumb (Adson) forceps | Fine Science Tools | 11006-12 | |
5 mL tubes, polypropylene | Falcon | 352063 | |
Tissue Ruptor disposable probes | Qiagen | 990890 | |
15 mL flip cap tubes | Thermo Scientific | 362694 | |
DNAse | Invitrogen | 18047019 | |
Liberase TM | Sigma-Aldrich | 5401119001 | should be aliquotted and stored at -20 °C to avoid repeated freeze-thaw |
100 µM MACS SmartStrainers | Miltenyi | 130-098-463 | compatible with 15 mL tubes, preferred |
100 µm cell strainers | Falcon | 352360 | compatible with 50 mL tubes, if MACS Smart Strainers are not available |
96 well plate | Falcon | 353077 | |
Fc block | BD Pharmingen | 553142 | anti-CD16/CD32 |
anti-mouse CD45 antibody | BD Biosciences | 561487 | many different fluorescent conjugation options are available |
5 mL tubes polystyrene with filter cap | Falcon | 352235 | |
insulin syringe | Terumo | BS05M2913 | |
hand held homogenizer | Qiagen | 9001272 | TissueRuptor |
flow cytometer | BD Biosciences | N/A | Fortessa SORP |
Flow cytometry software | BD Biosciences | N/A | Diva |