High tissue density of FOXP3+ T cells is associated with clinical outcome in prostate cancer

Cell-mediated immunity may impact prostate cancer progression and has great therapeutic potential. Here, we investigated the clinical significance of the numeric density of regulatory T cells (Tregs) in prostate cancer as the presence of such cells in the tumour microenvironment has been linked to clinical outcome in other tumour entities. We detected Tregs by FOXP3 immunohistochemistry in 88.8% of 2002 prostate cancer specimens in tissue microarray format, the largest cohort so far in which Tregs have been quantified. The density of Tregs in tumour tissue was compared with pathological parameters and clinical outcome. The number of Tregs identified per 0.6 mm tissue spot ranged from 1 to 10 in normal and 1 to 103 FOXP3+ cells in tumour samples. Prostate-specific antigen (PSA) recurrence-free survival was significantly reduced in patients with higher numbers of Tregs (p = 0.0151). Further, a higher number of intratumoural FOXP3+ Tregs was associated with a more advanced tumour stage (p = 0.0355) and higher Ki67 labelling index (p < 0.0001). The tissue density of Tregs was unrelated to other clinical parameters such as spread to lymph nodes, preoperative PSA level and Gleason score. Our study suggests that the intratumoural presence of regulatory T cells may have substantial functional impact and may confer an adverse clinical course in prostate cancer.