Introduction: The status of tumor-infiltrating lymphocytes (TILs) has been recently proposed to predict clinical outcome of patients with breast cancer. We therefore studied the prognostic significance of CD8+ TILs and FOXP3+ TILs in residual tumors after neoadjuvant chemotherapy (NAC) and the alterations in these parameters before and after NAC in patients with triple-negative breast cancer (TNBC). Methods: One hundred thirty-one TNBC patients who received NAC at three institutions were examined. CD8+ TIL and FOXP3+ TIL in residual tumors and biopsy specimens were evaluated by double-staining immunohistochemistry. The CD8+ TIL and FOXP3+ TIL status of the residual tumors was assessed, and the rates of their changes before and after NAC were calculated. Results: TNBC patients with high CD8+ TIL levels or a high CD8/FOXP3 ratio in residual tumors had significantly better recurrence-free survival (RFS) and breast cancer-specific survival (BCSS) than patients with low values of these parameters. In multivariate analyses, CD8+ TIL exhibited strong prognostic significance for RFS, with a hazard ratio (HR) of 3.09 (95 % confidence interval (CI) 1.537-6.614, P=0.0013). The CD8/FOXP3 ratio was also significantly correlated with RFS (HR=2.07, 95 % CI 1.029-4.436, P=0.0412). TNBC with larger residual tumor size and positive lymph node status, which are known prognostic factors, was independently associated with worse RFS (P=0.0064 and P=0.0015, respectively). High CD8+ TIL levels were a markedly powerful indicator of improved BCSS, with an HR of 3.59 (95 % CI 1.499-9.581, P=0.0036). Nodal status was also associated with BCSS (P=0.0024). TNBC with a high rate of CD8+ TIL changes was associated with significantly better RFS compared with the low group (P=0.011). Higher rates of changes in the CD8/FOXP3 ratio were significantly correlated with both better RFS and BCSS compared with lower rates (P=0.011 and P=0.023, respectively). Conclusions: This is the first study to demonstrate that high CD8+ TIL and a high CD8/FOXP3 ratio in residual tumors and increment of these parameters following NAC and accurately predict improved prognosis in TNBC patients with non-pathological complete response following NAC. These parameters could serve as a surrogate one for adjuvant treatment in patients with residual disease in the neoadjuvant setting.
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