To assess the clinical significance of tumor-infiltrating FOXP3-positive regulatory T cells (T_R) in breast cancer patients with long-term follow-up.

FOXP3-positive T_R were detected by immunohistochemistry with our new, extensively characterized FOXP3 monoclonal antibody, 236A/E7. Numbers of FOXP3-positive lymphocytes in tissue microarray cores from pure ductal carcinoma in situ (DCIS; n = 62), invasive breast cancer (n = 237) or from comparable areas of normal terminal duct lobular breast tissue (n = 10) were determined. A median cutoff of ≥ 15 defined patients with high numbers of T_R.

T_R numbers were significantly higher in in situ and invasive breast carcinomas than in normal breast; invasive tumors have significantly higher numbers than DCIS (P = .001). High numbers of FOXP3-positive T_R identified patients with DCIS at increased risk of relapse (P = .04) and patients with invasive tumors with both shorter relapse-free (P = .004) and overall survival (P = .007). High T_R numbers were present in high-grade tumors (P ≤ .001), in patients with lymph node involvement (P = .01), and in estrogen receptor (ER) –negative tumors (P = .001). Importantly, high numbers of T_R within ER-positive tumors identified high-risk patients (P = .005). Unlike conventional clinicopathologic factors, high numbers of FOXP3-positive T_R can identify patients at risk of relapse after 5 years.

These findings indicate that quantification of FOXP3-positive T_R in breast tumors is valuable for assessing disease prognosis and progression, and that T_R are an important therapeutic target for breast cancer. FOXP3-positive T_R represent a novel marker for identifying late-relapse patients who may benefit from aromatase therapy after standard tamoxifen treatment.