CCR4 Blockade Depletes Regulatory T Cells and Prolongs Survival in a Canine Model of Bladder Cancer

Regulatory T-cell (Treg) infiltration can be targeted as a cancer immunotherapy. Here, we describe therapeutic efficacy of this strategy in a canine model of bladder cancer. We used dogs with naturally occurring bladder cancer to study the molecular mechanism of Treg infiltration into bladder cancer tissues and the effect of anti-Treg treatment. Tumor-infiltrating Tregs were evaluated by immunohistochemistry, and their association with prognosis was examined in dogs with bladder cancer. The molecular mechanism of Treg infiltration was explored by RNA sequencing and protein analyses. Murine xenograft experiments and canine studies were used to explore the therapeutic potential of anti-Treg treatment for bladder cancer. We found that tumor-infil-trating Tregs were associated with poor prognosis in dogs bearing spontaneous bladder cancer. Treg infiltration was caused by interaction between the tumor-producing chemokine CCL17 and the receptor CCR4 expressed on Tregs. CCR4 blockade inhibited tumor growth and Treg infiltration into the tissues in a xenograft mouse model. Dogs with spontaneous bladder cancer responded to anti-CCR4 treatment with improved survival and low incidence of clinically relevant toxicities. In human patients with bladder cancer, immunohistochemistry showed that tumor-infiltrating Tregs expressed CCR4. Thus, anti-CCR4 treatment may be a rational approach to test in clinical trials for human patients with bladder cancer.