Tumour–Host Immunodynamics of Bacillus Calmette Guérin (BCG) Refractory Bladder Cancer

Introduction: Bladder cancer (BC) is dichotomized into muscle–invasive (MIBC) and non–muscle invasive (NMIBC), depending on tumour invasion across the lamina propria. Most BC patients are initially diagnosed with NMIBC. Intravesical administration of Bacillus Calmette-Guérin (BCG) has been considered the gold standard for high–risk NMIBC following tumour surgical removal. While BCG is established as a clinically beneficial immunotherapy, around 50% of patients recur or progress into the more aggressive MIBC. We aim to better understand the molecular underpinnings of BCG–mediated immune response profiles among responders and non–responders. Methods: Peripheral blood mononuclear cells were isolated from two blood draws of 16 patients with intermediate– and high–risk NMIBC, the first preceding treatment and the second following the fifth BCG installation (n = 32 samples). Patients were classified based on remission (n = 8) and recurrence (n = 8) within one year. Single–cell RNA sequencing determined immune cell type transcriptomic changes. Differential expression and pathway enrichment analyses were conducted to identify immune signatures differentiating responders from non–responders and the first (Week 1) versus last (Week 6) week of BCG management. Results: Regarding the transcriptome, an elevated metabolic response to BCG was detected in monocyte populations within responders. Particularly, mechanistic target of rapamycin complex I (mTORC1) signalling was upregulated in Week 6 compared to Week 1 within responders. Conversely, non–responders experienced downregulation of mTORC1 post–BCG. At Week 1, non–responders displayed a pre–inflammatory state that may underlie treatment resistance, marked by an upregulation of inflammatory pathways, ex. interferon (IF)–γ and tumour necrosis factor (TNF)–α. Conclusion: In NMIBC, non–responders to BCG immunotherapy display a preceding immune dysfunction characterized by metabolic and inflammatory disturbances. Pre–treatment peripheral immune profiling may effectively predict therapeutic outcomes and further personalize treatments for patients with early–stage NMIBC.