Background: Regulatory T cells (Tregs) play important roles in the suppression of immune responses, including antitumor immune responses. C-C chemokine receptor 4 (CCR4) is highly expressed on effector Tregs, and anti-CCR4 antibody is attracting attention as a novel immunotherapeutic agent for solid tumors. This study aimed to evaluate the expression of CCR4-positive Tregs (CCR4+Tregs) in prostate cancer and estimate the clinical potential of CCR4-targeting therapy for prostate cancer. Methods: A total of 15 radical prostatectomy (RP) specimens and 60 biopsy specimens from individuals diagnosed with prostate cancer were analyzed to evaluate the infiltration of CCR4+Tregs in prostate cancer. The relationships between the number of CCR4+Tregs and clinical parameters were investigated in RP and biopsy specimens. Moreover, the total number of Tregs, CCR4+Tregs, and T cells and the ratio of CCR4+Tregs to Tregs and T cells in biopsy specimens were compared between patients with poor prognosis who progressed to castration-resistant prostate cancer (CRPC) within 12 months (n = 13) and those with good prognosis who were stable with hormone-sensitive prostate cancer over 12 months (n = 47). Furthermore, biopsy specimens were divided into two groups: low and high CCR4+Treg expression groups and the prognosis was compared between them. Results: There was a higher expression of CCR4+Tregs in RP specimens with a higher (≥8) Gleason score than in those with a lower (<8) Gleason score (P =.041). In biopsy specimens, 65.9% Tregs were positive for CCR4. The number of CCR4+Tregs positively correlated with clinical stage (P <.001) and Gleason score (P =.006). The total number of Tregs and CCR4+Tregs significantly increased in the poor prognosis group compared with that in the good prognosis group (P =.024 and.01, respectively). Furthermore, patients with lower CCR4+Treg expression levels showed a significantly longer time to progression to CRPC (not reached vs 27.3 months; P <.001) and median survival time (not reached vs 69.0 months; P =.014) than those with higher expression levels. Conclusions: CCR4+Tregs are highly infiltrated in the prostate tissue of patients with poor prognosis with potential to progress to CRPC. Furthermore, the degree of infiltration of CCR4+Tregs is related to the prognosis of prostate cancer.
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