Eosinophils but not mast cells exert anti-tumorigenic activity, without being predictive markers of the long-term response to Bacillus Calmette-Guérin (BCG) therapy in patients with bladder carcinoma

Background: Bacillus Calmette-Guerin (BCG) therapy is an established immunotherapy for non-muscle invasive bladder cancer (NMIBC); however, the response variability of patients remains a challenge, necessitating insight into immune cell function. Previous studies established that a preexisting Th2 immune microenvironment correlates with a positive BCG therapy outcome. Therefore, in this study, we explored the role of mast cells (MCs) and eosinophils in bladder cancer as a potential predicting tool for BCG immunotherapy response. Methods: We investigated the effect of MCs and eosinophils on bladder cancer cell viability together with their chemotactic migration towards cancer cells in vitro. The effect of BCG on these immune cells was also evaluated. Moreover, we performed an orthotopic model of bladder cancer in MC- and eosinophil-deficient mice. Finally, to evaluate whether these immune cells predict the therapy response, 26 patient biopsies pre-BCG treatment were analyzed for MC and eosinophil numbers in the tissue and sequenced for gene expression. Results: Eosinophils, but not MCs, reduced bladder cancer cell viability in vitro and inhibited tumor growth in vivo. However, addition of BCG did not increase these effects in vitro. Patient biopsy analysis and mRNA sequencing showed that neither cell type predicted long-term therapy responsiveness. Gene expression analysis suggested that extracellular matrix and epithelial-to-mesenchymal transition factors could influence BCG therapy outcomes. Conclusion: Even though eosinophils exhibit anti-tumorigenic effects in bladder cancer, neither MCs nor eosinophils were predictive of the long-term BCG therapy response. However, our findings implicate that matrix-related factors may modulate BCG therapy responses.