Background and Objectives: To explore whether lymphocytes in sentinel lymph nodes (SLNs) are highly exposed to tumor neoantigens and thus express high level of programmed death 1 (PD-1), we examined PD-1 expression in SLNs and non-sentinel regional lymph nodes (non-SLNs) in breast cancer. Methods: We performed PD-1 immunohistochemistry in two cohorts: 40 metastasis-negative SLNs including 10 patients for each subtype (luminal A-like, luminal B-like, HER2, and triple negative breast cancer [TNBC]); and 25 pairs of metastasis-positive SLNs and non-SLNs (10 luminal A-like, 10 luminal B-like, and 5 TNBC). Results: Among 40 metastasis-negative SLNs, 34 and 6 samples were PD-1 intensity grade 1 (low) and 2 (high), respectively. PD-1 intensity correlated with PD-1-positive lymphocyte numbers (P = 0.005); TNBC had the highest PD-1 lymphocyte numbers among all subtypes. The median PD-1-positive lymphocyte number was higher in SLNs than non-SLNs. In most cases, more lymphocytes in SLNs expressed PD-1 than those in non-SLNs (P < 0.0001). Conclusions: TNBC had the greatest PD-1 expression among all subtypes, and metastasis-positive SLNs had more PD-1-positive lymphocytes than downstream non-SLNs. These data suggested that lymphocytes in SLNs are activated following exposure to tumor neoantigens and thus tumor specific, and could be utilized as a biomarker platform.
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