Non-muscle invasive bladder cancer (NMIBC) is typically treated by local tumor resection followed by intravesical (instilled into the bladder) Bacillus Calmette-Guerin (BCG) immunotherapy. BCG is the most effecting treatment for NMIBC and is the gold standard treatment for immunocompetent patients. Unfortunately, up to 70% of individuals undergoing BCG therapy experience lower urinary tract symptoms (LUTS), characterized by symptoms such as bladder pain, increased urinary urgency and frequency. These LUTS can significantly impact quality of life, with reports of 7-20% of individuals stopping BCG treatment due to the severity of these symptoms. 

Bladder sensation is mediated by the activation of bladder-innervating sensory (afferent) nerves and it is well known that an inflammatory state can induce bladder afferent hypersensitivity. As the primary mechanism behind BCG immunotherapy is to induce an intense localized immune response in the bladder we hypothesized that the side effects of BCG-therapy are related to bladder afferent hypersensitivity. 

The aim of this study was to determine the mechanisms underlying the development of BCG-induced LUTS.

To determine the underlying mechanisms of BCG induced LUTS we developed a pre-clinical model of BCG-therapy in C57BL/6 female mice. Female mice received once weekly instillations of BCG (2.5x108CFU/ml) for 6 consecutive weeks, SHAM mice received once weekly saline instillations. One day after the final infusion, we assessed the development of inflammation via RNAsequencing and flow cytometry of bladder single cell suspensions (N=10/group). Bladder sensory signalling was assessed utilising an ex-vivo bladder afferent nerve recording technique during bladder distension (0-50mmHg) (N=5/group). Referred pelvic pain was measured in vivo as the withdrawal threshold to electronic von-Frey hair (eVFH) stimulation of the pelvic region (N=10/group).

We show that our mouse model of BCG immunotherapy develops significant inflammation, characterised by significant upregulation of key Hallmark inflammatory pathways, and a significant increase in the number of infiltrating CD45+ immune cells (P<0.01, N=10/group) including, dendritic cells (P<0.05, inflammatory monocytes (P<0.001), natural killer cells (P<0.001) and CD4+ (P<0.001) and CD8+ T cells (P<0.05) (Figure 1). Ex-vivo afferent recordings revealed BCG-treated mice develop bladder afferent sensitivity to distension (P<0.0001, N=5/group), with the sensitisation of both low (P<0.01) and high threshold (P<0.01) single unit afferents. Mice also develop pelvic pain, as indicated by a significant decrease in the withdrawal threshold to pelvic eVFH stimulation (P<0.05, N=10/group).

Our study reveals that BCG-treatment in mice induces inflammation, bladder afferent hypersensitivity, and pelvic pain. Bladder afferent hypersensitivity is a common feature underlying the development of multiple bladder disorders characterized by LUTS. Our study indicates bladder afferent hypersensitivity may also underlie the development of BCG-induced LUTS.

BCG treated mice develop inflammation, bladder afferent hypersensitivity, and pelvic pain. Targeting hypersensitive bladder afferents may represent a novel therapeutic target for the relief of BCG-induced LUTS to increase patient compliance for better outcomes and improved patient wellbeing.