Abstract:

Urinary tract infections (UTI) are one of the most common bacterial infections worldwide, affecting over 400 million people every year. The incidence of infection greatly varies with 50% of woman and only 5% of men experiencing one UTI in their lifetime. Women tend to experience recurrent UTI, whereas men develop chronic UTI. We can recapitulate human infection outcomes with a mouse model of UTI in which uropathogenic Escherichia coli (UPEC) is inoculated transurethrally into the bladder. Female mice clear their infection spontaneously in a week, whereas male mice remain infected at least 1 month post-infection (PI). Although bacterial burden peaks at 24 hours PI in both sexes, at this timepoint, female mice exhibit more robust innate immune cell infiltration and cytokine expression compared to male mice. Notably, this is not the case in the first hour after infection, in which we found that female mice had more bacteria in their bladders compared to male mice. I found that the transcription factor NFkB was activated before infection and peaked earlier in infection in female urothelial cells compared to males. This suggests the first hours of infection are essential for initiating a strong immune response to promote resolution.

To more closely understand the early response, I performed RNA sequencing from peeled bladder urothelium, finding higher immune gene expression in female mice compared to male mice at 1 hour PI. In contrast to females, in male mice, many metabolic pathways were upregulated, suggesting males failed to activate the glycolytic switch typically associated with innate immune responses, which may delay their immune response and possibly, infection outcomes. Pathway activity analysis predicted that the NFkB pathway and epidermal growth factor receptor (EGFR) pathway were activated at one hour PI in female mice. I observed that EGFR signaling had a dual function as activating the receptor increased overall immune cell infiltration 24 hours PI in female mice and induced an anti-inflammatory response to initiate repair in both sexes. Upstream of these pathways, I found that TLR4 impacted bacterial colonization at 1 hour PI, effective resolution of infection, and robustness of the immune response. In the absence of TLR4, female mice had lower bacterial invasion and impaired bacterial clearance.

These findings currently suggest that TLR4 is the factor driving the difference in innate immune responses between the sexes. Whether TLR4 directly mediates invasion is unclear. Indeed, TLR4 may support invasion and downstream signaling of infection only in females, or TLR4 may be inaccessible for bacterial sensing in males. I found a sexually dimorphic sugar-dense layer on the surface of the urothelium, which may potentially impact bacterial interaction and invasion at this surface. Delayed invasion and lack of engagement of TLR4 would contribute to the attenuation of the male immune response in the bladder.

Finally, in projects related to my main investigations, I co-developed an ex vivo bladder infection model, in which we can study early host-pathogen interactions. I quantified the distribution of immune cells in the urothelium and demonstrated that urothelial and immune cells take up bacteria very rapidly. This model will be important to test our hypotheses regarding TLR4. In related in vivo studies, I investigated sexually dimorphic macrophage subsets, gd T cells, and sex differences in young and aged mice. Finally, I investigated key aspects of UTI in sex-specific organs, the prostate and uterus. Both organs are colonized by UPEC as early as 1 hour PI and have both similarities and specific distinctions in their immune responses to infection. My findings will help identify essential regulators of the immune response between the sexes, which will drive the development of ‘tailor-made’ treatment for women and men in the clinic in the age of antibiotic-resistance.

Keywords : urinary tract infection, sex differences, innate immunity, mucosal immunity, nuclear factor kappa B, epidermal growth factor receptor, toll-like receptor pathway, mucus layer