CCR8 blockade primes anti-tumor immunity through intratumoral regulatory T cells destabilization in muscle-invasive bladder cancer

Regulatory T cells (Tregs) play a major role in the development of an immunosuppressive tumor microenvironment. Systemic Treg depletion is not favored because of the critical role of Tregs in maintaining immune homeostasis and preventing the autoimmunity. Recently, CCR8 has been identified as an important chemokine receptor expressed on intratumoral Tregs and is known to be critical for CCR8(+)Treg-mediated immunosuppression. However, the inherent molecular mechanisms and clinical significance of intratumoral CCR8(+)Tregs remain poorly understood. In this study, a retrospective analysis of 259 muscle-invasive bladder cancer (MIBC) patients from two independent clinic centers was conducted to explore the prognostic merit of CCR8(+)Tregs via immunohistochemistry. Eighty-three fresh MIBC samples and data from the Cancer Genome Atlas were used to evaluate the proportion and function of immune cells via flow cytometry, ex vivo intervention experiments and bioinformatics analysis. It was found that the CCR8 expression by intratumoral Tregs maintained the stability and potentiated their suppressive function by upregulating the expression of transcript factors FOXO1 and c-MAF. High level of CCR8(+)Tregs was associated with the immune tolerance and predicted poor survival and inferior therapeutic responsiveness to chemotherapy. Moreover, it was revealed that CCR8 blockade could destabilize intratumoral Tregs into a fragile phenotype accompanied with reactivation of antitumor immunity and augment of anti-PD-1 therapeutic benefits in MIBC. In summary, those results suggested that CCR8(+)Tregs represented a stable Treg subtype and a promising therapeutic target in the immunotherapy of MIBC.