Intravesical BCG Induces CD4⁺ T-cell expansion in an immune competent model of bladder cancer

Intravesical bacillus Calmette-Guérin (BCG) immunotherapy is the standard of care in treating non-muscle-invasive bladder cancer, yet its mechanism of action remains elusive. Both innate and adaptive immune responses have been implicated in BCG activity. Although prior research has indirectly demonstrated the importance of T cells and shown a rise in CD4⁺ T cells in bladder tissue after BCG, T-cell subpopulations have not been fully characterized. We investigated the relationship between effector and regulatory T cells in an immune competent, clinically relevant rodent model of bladder cancer. Our data demonstrate that cancer progression in the N-methyl-N-nitrosourea (MNU) rat model of bladder cancer was characterized by a decline in the CD8/FoxP3 ratio, consistent with decreased adaptive immunity. In contrast, treatment with intravesical BCG led to a large, transient rise in the CD4⁺ T-cell population in the urothelium and was both more effective and immunogenic compared with intravesical chemotherapy. Whole-transcriptome expression profiling of posttreatment intravesical CD4⁺ and CD8⁺ T cells revealed minimal differences in gene expression after BCG treatment. Together, our results suggest that although BCG induces T-cell recruitment to the bladder, the T-cell phenotype does not markedly change, implying that combining T-cell-activating agents withBCGmight improve clinical activity. Cancer Immunol Res; 5(7); 594-603.